index.js perf: remove .forEach in Multi.prototype.exec

I think there needs to be a better way to compare the performance before and after the change.

It's difficult to look at the numbers and be sure that it had a positive influence. Probably should have used a profiler first to see how hot the code path is — should have thought of that earlier.

**I don't think this is ready for merging** — I need to write a little script that examines the differences between the `node multibench.js` results for each run.

.forEach loops (5351 ms test.js):

```
Client count: 5, node version: 0.6.12, server version: 2.4.8, parser: hiredis
         PING,     1/5 min/max/avg/p95:    0/   6/   0.10/   1.00   2101ms total,  9519.28 ops/sec
         PING,    50/5 min/max/avg/p95:    0/  15/   1.66/   3.00    667ms total, 29985.01 ops/sec
         PING,   200/5 min/max/avg/p95:    1/  24/   6.30/  10.00    632ms total, 31645.57 ops/sec
         PING, 20000/5 Client count: 5, node version: 0.6.12, server version: 2.4.8, parser: hiredis
         PING,     1/5 min/max/avg/p95:    0/   6/   0.10/   1.00   2106ms total,  9496.68 ops/sec
         PING,    50/5 min/max/avg/p95:    0/  19/   1.76/   3.00    705ms total, 28368.79 ops/sec
         PING,   200/5 min/max/avg/p95:    1/  22/   6.32/  11.00    635ms total, 31496.06 ops/sec
         PING, 20000/5 min/max/avg/p95:   56/ 598/ 320.38/ 550.00    621ms total, 32206.12 ops/sec
SET small str,     1/5 min/max/avg/p95:    0/  19/   0.11/   1.00   2220ms total,  9009.01 ops/sec
SET small str,    50/5 min/max/avg/p95:    0/  34/   1.81/   3.00    728ms total, 27472.53 ops/sec
SET small str,   200/5 min/max/avg/p95:    0/  17/   6.51/  11.00    653ms total, 30627.87 ops/sec
SET small str, 20000/5 min/max/avg/p95:   51/ 638/ 351.74/ 578.10    648ms total, 30864.20 ops/sec
SET small buf,     1/5 min/max/avg/p95:    0/  19/   0.16/   1.00   3296ms total,  6067.96 ops/sec
SET small buf,    50/5 min/max/avg/p95:    0/  27/   5.10/   8.00   2043ms total,  9789.53 ops/sec
SET small buf,   200/5 min/max/avg/p95:    0/  68/  20.77/  34.00   2087ms total,  9583.13 ops/sec
SET small buf, 20000/5 min/max/avg/p95:  203/2100/1177.03/2003.20   2109ms total,  9483.17 ops/sec
GET small str,     1/5 min/max/avg/p95:    0/   7/   0.10/   1.00   2093ms total,  9555.66 ops/sec
GET small str,    50/5 min/max/avg/p95:    0/  30/   1.68/   3.00    675ms total, 29629.63 ops/sec
GET small str,   200/5 min/max/avg/p95:    0/  22/   6.37/  10.00    639ms total, 31298.90 ops/sec
GET small str, 20000/5 min/max/avg/p95:   52/ 626/ 341.20/ 588.55    662ms total, 30211.48 ops/sec
GET small buf,     1/5 min/max/avg/p95:    0/   7/   0.10/   1.00   2084ms total,  9596.93 ops/sec
GET small buf,    50/5 min/max/avg/p95:    0/  23/   1.65/   3.00    663ms total, 30165.91 ops/sec
GET small buf,   200/5 min/max/avg/p95:    1/  31/   6.97/  12.00    700ms total, 28571.43 ops/sec
GET small buf, 20000/5 min/max/avg/p95:   51/ 682/ 351.47/ 616.55    693ms total, 28860.03 ops/sec
SET large str,     1/5 min/max/avg/p95:    0/  21/   0.14/   1.00   2920ms total,  6849.32 ops/sec
SET large str,    50/5 min/max/avg/p95:    0/  27/   3.31/   5.00   1326ms total, 15082.96 ops/sec
SET large str,   200/5 min/max/avg/p95:    1/  44/  12.92/  33.00   1297ms total, 15420.20 ops/sec
SET large str, 20000/5 min/max/avg/p95:  202/1715/ 961.46/1592.10   1748ms total, 11441.65 ops/sec
SET large buf,     1/5 min/max/avg/p95:    0/  16/   0.18/   1.00   3605ms total,  5547.85 ops/sec
SET large buf,    50/5 min/max/avg/p95:    0/  17/   5.06/   8.00   2026ms total,  9871.67 ops/sec
SET large buf,   200/5 min/max/avg/p95:    0/  44/  19.95/  32.00   2005ms total,  9975.06 ops/sec
SET large buf, 20000/5 min/max/avg/p95:  200/2180/1220.56/2088.00   2189ms total,  9136.59 ops/sec
GET large str,     1/5 min/max/avg/p95:    0/  20/   0.11/   1.00   2346ms total,  8525.15 ops/sec
GET large str,    50/5 min/max/avg/p95:    0/  19/   2.80/   6.00   1121ms total, 17841.21 ops/sec
GET large str,   200/5 min/max/avg/p95:    0/  46/   9.53/  24.00    958ms total, 20876.83 ops/sec
GET large str, 20000/5 min/max/avg/p95:  422/ 831/ 583.07/ 765.55   1130ms total, 17699.12 ops/sec
GET large buf,     1/5 min/max/avg/p95:    0/  12/   0.11/   1.00   2348ms total,  8517.89 ops/sec
GET large buf,    50/5 min/max/avg/p95:    0/  16/   2.45/   4.00    988ms total, 20242.91 ops/sec
GET large buf,   200/5 min/max/avg/p95:    0/  33/   9.38/  20.00    942ms total, 21231.42 ops/sec
GET large buf, 20000/5 min/max/avg/p95:  361/ 691/ 548.64/ 659.55   1036ms total, 19305.02 ops/sec
         INCR,     1/5 min/max/avg/p95:    0/  17/   0.11/   1.00   2154ms total,  9285.05 ops/sec
         INCR,    50/5 min/max/avg/p95:    0/  20/   1.71/   3.00    685ms total, 29197.08 ops/sec
         INCR,   200/5 min/max/avg/p95:    1/  30/   6.62/  12.00    665ms total, 30075.19 ops/sec
         INCR, 20000/5 min/max/avg/p95:   48/ 638/ 354.40/ 595.00    647ms total, 30911.90 ops/sec
        LPUSH,     1/5 min/max/avg/p95:    0/   7/   0.10/   1.00   2099ms total,  9528.35 ops/sec
        LPUSH,    50/5 min/max/avg/p95:    0/  25/   1.70/   3.00    681ms total, 29368.58 ops/sec
        LPUSH,   200/5 min/max/avg/p95:    0/  40/   6.55/  10.00    658ms total, 30395.14 ops/sec
        LPUSH, 20000/5 min/max/avg/p95:   49/ 631/ 355.67/ 596.10    641ms total, 31201.25 ops/sec
    LRANGE 10,     1/5 min/max/avg/p95:    0/  19/   0.12/   1.00   2436ms total,  8210.18 ops/sec
    LRANGE 10,    50/5 min/max/avg/p95:    0/  20/   1.95/   3.00    782ms total, 25575.45 ops/sec
    LRANGE 10,   200/5 min/max/avg/p95:    0/  33/   8.59/  19.00    863ms total, 23174.97 ops/sec
    LRANGE 10, 20000/5 min/max/avg/p95:   89/ 722/ 398.88/ 664.10    754ms total, 26525.20 ops/sec
   LRANGE 100,     1/5 min/max/avg/p95:    0/  16/   0.21/   1.00   4231ms total,  4727.01 ops/sec
   LRANGE 100,    50/5 min/max/avg/p95:    1/  17/   4.33/   8.00   1735ms total, 11527.38 ops/sec
   LRANGE 100,   200/5 min/max/avg/p95:    3/  45/  18.99/  30.00   1906ms total, 10493.18 ops/sec
   LRANGE 100, 20000/5 min/max/avg/p95:  668/2226/1713.18/2147.00   2478ms total,  8071.03 ops/sec
End of tests.
```

for loops (5217 ms test.js):

```
Client count: 5, node version: 0.6.12, server version: 2.4.8, parser: hiredis
         PING,     1/5 min/max/avg/p95:    0/   8/   0.11/   1.00   2188ms total,  9140.77 ops/sec
         PING,    50/5 min/max/avg/p95:    0/  17/   1.95/   4.00    785ms total, 25477.71 ops/sec
         PING,   200/5 min/max/avg/p95:    1/  25/   6.69/  11.00    678ms total, 29498.53 ops/sec
         PING, 20000/5 min/max/avg/p95:   52/ 600/ 319.67/ 552.55    621ms total, 32206.12 ops/sec
SET small str,     1/5 min/max/avg/p95:    0/  19/   0.11/   1.00   2188ms total,  9140.77 ops/sec
SET small str,    50/5 min/max/avg/p95:    0/  26/   1.75/   3.00    700ms total, 28571.43 ops/sec
SET small str,   200/5 min/max/avg/p95:    0/  18/   6.49/  11.00    652ms total, 30674.85 ops/sec
SET small str, 20000/5 min/max/avg/p95:   49/ 669/ 358.86/ 626.55    679ms total, 29455.08 ops/sec
SET small buf,     1/5 min/max/avg/p95:    0/  18/   0.17/   1.00   3416ms total,  5854.80 ops/sec
SET small buf,    50/5 min/max/avg/p95:    1/  26/   5.49/  10.00   2199ms total,  9095.04 ops/sec
SET small buf,   200/5 min/max/avg/p95:    1/  55/  20.76/  33.00   2085ms total,  9592.33 ops/sec
SET small buf, 20000/5 min/max/avg/p95:  194/2166/1216.54/2049.55   2176ms total,  9191.18 ops/sec
GET small str,     1/5 min/max/avg/p95:    0/   5/   0.10/   1.00   2160ms total,  9259.26 ops/sec
GET small str,    50/5 min/max/avg/p95:    0/  29/   1.80/   3.00    722ms total, 27700.83 ops/sec
GET small str,   200/5 min/max/avg/p95:    0/  41/   6.74/  11.00    678ms total, 29498.53 ops/sec
GET small str, 20000/5 min/max/avg/p95:   54/ 635/ 340.72/ 584.55    646ms total, 30959.75 ops/sec
GET small buf,     1/5 min/max/avg/p95:    0/   5/   0.10/   1.00   2155ms total,  9280.74 ops/sec
GET small buf,    50/5 min/max/avg/p95:    0/  23/   1.75/   3.00    703ms total, 28449.50 ops/sec
GET small buf,   200/5 min/max/avg/p95:    0/  32/   6.57/  10.00    660ms total, 30303.03 ops/sec
GET small buf, 20000/5 min/max/avg/p95:   50/ 644/ 357.94/ 601.00    654ms total, 30581.04 ops/sec
SET large str,     1/5 min/max/avg/p95:    0/  30/   0.14/   1.00   2891ms total,  6918.02 ops/sec
SET large str,    50/5 min/max/avg/p95:    0/  29/   3.36/   6.00   1347ms total, 14847.81 ops/sec
SET large str,   200/5 min/max/avg/p95:    0/  65/  13.60/  35.55   1368ms total, 14619.88 ops/sec
SET large str, 20000/5 min/max/avg/p95:  176/1743/1051.82/1640.10   1835ms total, 10899.18 ops/sec
SET large buf,     1/5 min/max/avg/p95:    0/  42/   0.18/   1.00   3722ms total,  5373.46 ops/sec
SET large buf,    50/5 min/max/avg/p95:    1/  31/   5.25/   8.00   2103ms total,  9510.22 ops/sec
SET large buf,   200/5 min/max/avg/p95:    1/  53/  20.40/  33.00   2049ms total,  9760.86 ops/sec
SET large buf, 20000/5 min/max/avg/p95:  404/2617/1550.55/2492.55   2626ms total,  7616.15 ops/sec
GET large str,     1/5 min/max/avg/p95:    0/  29/   0.12/   1.00   2448ms total,  8169.93 ops/sec
GET large str,    50/5 min/max/avg/p95:    0/  16/   2.44/   4.00    978ms total, 20449.90 ops/sec
GET large str,   200/5 min/max/avg/p95:    0/  41/   9.63/  21.00    966ms total, 20703.93 ops/sec
GET large str, 20000/5 min/max/avg/p95:  341/ 773/ 560.95/ 735.55   1063ms total, 18814.68 ops/sec
GET large buf,     1/5 min/max/avg/p95:    0/  12/   0.11/   1.00   2354ms total,  8496.18 ops/sec
GET large buf,    50/5 min/max/avg/p95:    0/  18/   2.45/   4.00    980ms total, 20408.16 ops/sec
GET large buf,   200/5 min/max/avg/p95:    0/  53/  10.61/  24.00   1065ms total, 18779.34 ops/sec
GET large buf, 20000/5 min/max/avg/p95:  293/ 723/ 548.55/ 699.00   1053ms total, 18993.35 ops/sec
         INCR,     1/5 min/max/avg/p95:    0/  17/   0.10/   1.00   2117ms total,  9447.33 ops/sec
         INCR,    50/5 min/max/avg/p95:    0/  23/   1.67/   3.00    669ms total, 29895.37 ops/sec
         INCR,   200/5 min/max/avg/p95:    0/  17/   6.28/  10.00    630ms total, 31746.03 ops/sec
         INCR, 20000/5 min/max/avg/p95:   51/ 604/ 335.57/ 559.55    614ms total, 32573.29 ops/sec
        LPUSH,     1/5 min/max/avg/p95:    0/  20/   0.10/   1.00   2116ms total,  9451.80 ops/sec
        LPUSH,    50/5 min/max/avg/p95:    0/  25/   1.70/   3.00    681ms total, 29368.58 ops/sec
        LPUSH,   200/5 min/max/avg/p95:    1/  31/   6.43/  10.00    646ms total, 30959.75 ops/sec
        LPUSH, 20000/5 min/max/avg/p95:   49/ 646/ 364.86/ 598.00    657ms total, 30441.40 ops/sec
    LRANGE 10,     1/5 min/max/avg/p95:    0/   5/   0.12/   1.00   2455ms total,  8146.64 ops/sec
    LRANGE 10,    50/5 min/max/avg/p95:    0/  23/   2.00/   3.00    801ms total, 24968.79 ops/sec
    LRANGE 10,   200/5 min/max/avg/p95:    1/  29/   7.39/  12.00    742ms total, 26954.18 ops/sec
    LRANGE 10, 20000/5 min/max/avg/p95:   62/ 727/ 396.63/ 665.00    748ms total, 26737.97 ops/sec
   LRANGE 100,     1/5 min/max/avg/p95:    0/  22/   0.20/   1.00   4056ms total,  4930.97 ops/sec
   LRANGE 100,    50/5 min/max/avg/p95:    0/  28/   4.57/   8.00   1831ms total, 10922.99 ops/sec
   LRANGE 100,   200/5 min/max/avg/p95:    2/  55/  20.34/  34.00   2040ms total,  9803.92 ops/sec
   LRANGE 100, 20000/5 min/max/avg/p95:  767/2185/1610.75/2060.55   2395ms total,  8350.73 ops/sec
End of tests.
```