MGET improve

hi, @manjuraj, I'm trying to import mget performance by rewrite it like this::

orig: 

    mget k1 k2 k3 k4 k5 k6 k7 k8 k9 

after rewrite:

    mget k1 k3 k6
    mget k2 k4 k7
    mget k5 k8 k9

this will cost less time on ``req_done()`` function calls, 

I use this redis-benchmark::

        if (test_is_selected("mget")) { 
    #define N 1000
            const char *argv[N+1];
            argv[0] = "MGET";
            for (i = 1; i < N+1; i += 1) {
                argv[i] = "key:__rand_int__";
            }
            len = redisFormatCommandArgv(&cmd,N+1,argv,NULL);
            char tmp[1024];
            sprintf(tmp, "MGET (%d keys)", N);
            benchmark(tmp, cmd,len);
            free(cmd);
        }

here is the  benchmark result  on my laptop:

1000 keys mget::

    ning@ning-laptop:~/idning-github/redis-mgr$ redis-benchmark -n 1000 -t mget -p 4001 -r 100000000
    ====== MGET (1000 keys) ======
      1000 requests completed in 21.66 seconds
      50 parallel clients
      3 bytes payload
      keep alive: 1

    100.00% <= 1268 milliseconds
    46.18 requests per second

    ning@ning-laptop:~/idning-github/redis-mgr$ redis-benchmark -n 1000 -t mget -p 4000 -r 100000000
    ====== MGET (1000 keys) ======
      1000 requests completed in 1.64 seconds
      50 parallel clients
      3 bytes payload
      keep alive: 1

    100.00% <= 102 milliseconds
    611.25 requests per second  (improved )

    (benchmark against redis got 1292 requests per second)

100keys mget::

    ning@ning-laptop:~/idning-github/redis-mgr$ redis-benchmark -n 10000 -t mget -p 4001 -r 100000000
    ====== MGET (100 keys) ======
      10000 requests completed in 8.57 seconds
      50 parallel clients
      3 bytes payload
      keep alive: 1

    100.00% <= 56 milliseconds
    1166.59 requests per second

    ning@ning-laptop:~/idning-github/redis-mgr$ redis-benchmark -n 10000 -t mget -p 4000 -r 100000000
    ====== MGET (100 keys) ======
      10000 requests completed in 1.88 seconds
      50 parallel clients
      3 bytes payload
      keep alive: 1

    100.00% <= 17 milliseconds
    5316.32 requests per second  (improved )

    (benchmark against redis got 11668.61 requests per second)

the performance is about ``5x-10x`` (about 1/2 of redis instance) 
@neeleshkorade  will be happy to see this result (https://github.com/twitter/twemproxy/issues/158)

I need a detail benchmark later. and I need your advise. 


get_mbuf is also used in twemproxy_auth_and_select branch. I think we will merge it later. so I keep this a function. 



https://github.com/twitter/twemproxy/blob/twemproxy_auth_and_select/src/nc_request.c#L383-L400

and we got smaller ``msg_make_reply`` function body.