Add sibling->regular entry conversion sweeper, to GC keydir siblings

The amortization mechanism attempts to limit itself to less than
1 msec of additional latency for any get/put/delete call, but as
shown below, it doesn't always stay strictly under that limit when
you're freeing hundreds of megabytes of data.

Below are histograms showing the NIF function latency as recorded
on OS X on my MBP for a couple of different mutation rates: 90%
and 9%.  The workload is:

* Put 10M keys
* Create an iterator
* Modify some percentage of the keys (e.g. 90%, 9%)
* Close the iterator
* Fetch all 10M keys, measuring the NIF latency time of each call.

---- snip ---- snip ---- snip ---- snip ---- snip ----

By my back-of-the-envelope calculations (1 word = 8 bytes)

      bitcask_keydir_entry = 3 words + key bytes
    vs.
      bitcask_keydir_entry_head = 2 words + key bytes
      plus
      bitcask_keydir_entry_sib = 5 words

So each 2-sibling entry is using 2 + (2*5) = 12 words, not counting
the key bytes.
After the sibling sweep, we're going from 12 words -> 3 words per key.

So for 10M keys, that's a savings of 9 words -> 687MBytes.

(RSS for the 10M keys @ 90% mutation tests peaks at 1.45GB of RAM.  By
 comparison, the 10M key @ 0% mutation test peaks at ~850MByte RSS.
 So, those numbers roughly match, yay.)

No wonder that there are a very small number of
bitcask_nifs_keydir_get_int() calls that are taking > 10msec to
finish: it may be the OS getting involved with side-effects from
free(3) calls??

10M keys, ~90% mutated, 0% deleted
via:
    bitcask_nifs:yoo(10*1000000, 9*1000*1000, 0).

    bitcask_nifs_keydir_get_int latency with off-cpu (usec)
             value  ------------- Distribution ------------- count
                 8 |                                         0
                16 |@@@@@@@@@@@@@@@                          40
                32 |@@@@@@                                   17
                64 |@@@                                      8
               128 |@@                                       5
               256 |@@                                       5
               512 |@@@@@@@@@@                               27
              1024 |@@                                       5
              2048 |                                         0

    bitcask_nifs_keydir_get_int latency (usec)
             value  ------------- Distribution ------------- count
                 0 |                                         0
                 1 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@         7954469
                 2 |@@@@@@@@                                 2051656
                 4 |                                         10440
                 8 |                                         25446
                16 |                                         209
                32 |                                         5
                64 |                                         0
               128 |                                         9
               256 |                                         0
               512 |                                         11698
              1024 |                                         723
              2048 |                                         12
              4096 |                                         0
              8192 |                                         0
             16384 |                                         1
             32768 |                                         1
             65536 |                                         0

    bitcask_nifs_keydir_get_int latency with off-cpu (usec)
             value  ------------- Distribution ------------- count
                 8 |                                         0
                16 |@@@@@@@@@@@@@@@@@@@@@@@@@@@              56
                32 |@@@@@@                                   13
                64 |@@@@                                     8
               128 |@@                                       5
               256 |                                         0

    bitcask_nifs_keydir_get_int latency (usec)
             value  ------------- Distribution ------------- count
                 0 |                                         0
                 1 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@          7754589
                 2 |@@@@@@@@@                                2211364
                 4 |                                         15906
                 8 |                                         25269
                16 |                                         375
                32 |                                         8
                64 |                                         0

10M keys, ~9% mutated, 0% deleted
via:
    bitcask_nifs:yoo(10*1000000, 9 * 100*1000, 0).

    bitcask_nifs_keydir_get_int latency with off-cpu (usec)
             value  ------------- Distribution ------------- count
                 8 |                                         0
                16 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@             64
                32 |@@@@@                                    12
                64 |@@@                                      7
               128 |@@                                       4
               256 |@                                        3
               512 |@                                        2
              1024 |                                         0

    bitcask_nifs_keydir_get_int latency (usec)
             value  ------------- Distribution ------------- count
                 0 |                                         0
                 1 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@           7530608
                 2 |@@@@@@@@@@                               2465556
                 4 |                                         12014
                 8 |                                         24721
                16 |                                         234
                32 |                                         6
                64 |                                         0
               128 |                                         0
               256 |                                         1
               512 |                                         1473
              1024 |                                         3
              2048 |                                         0

    bitcask_nifs_keydir_get_int latency with off-cpu (usec)
             value  ------------- Distribution ------------- count
                 4 |                                         0
                 8 |@                                        3
                16 |@@@@@@@@@@@@@@@@@@@@@@@@@                55
                32 |@@@@@@@                                  15
                64 |@@                                       5
               128 |@@@                                      6
               256 |@                                        2
               512 |                                         1
              1024 |                                         0

    bitcask_nifs_keydir_get_int latency (usec)
             value  ------------- Distribution ------------- count
                 0 |                                         0
                 1 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@           7631794
                 2 |@@@@@@@@@                                2345179
                 4 |                                         13769
                 8 |                                         26056
                16 |                                         324
                32 |                                         22
                64 |                                         2
               128 |                                         0