Fix concurrency issue in CompactionPicker

Summary:
I am currently working on a project that uses RocksDB. While debugging some perf issues, I came up across interesting compaction concurrency issue. Namely, I had 15 idle threads and a good comapction to do, but CompactionPicker returned "Compaction nothing to do". Here's how Internal stats looked:

    2014/08/22-08:08:04.551982 7fc7fc3f5700 ------- DUMPING STATS -------
    2014/08/22-08:08:04.552000 7fc7fc3f5700
    ** Compaction Stats [default] **
    Level   Files   Size(MB) Score Read(GB)  Rn(GB) Rnp1(GB) Write(GB) Wnew(GB) RW-Amp W-Amp Rd(MB/s) Wr(MB/s)  Rn(cnt) Rnp1(cnt) Wnp1(cnt) Wnew(cnt)  Comp(sec) Comp(cnt) Avg(sec) Stall(sec) Stall(cnt) Avg(ms)
    ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
      L0     7/5        353   1.0      0.0     0.0      0.0       2.3      2.3    0.0   0.0      0.0      9.4        0         0         0         0        247        46    5.359       8.53          1 8526.25
      L1     2/2         86   1.3      2.6     1.9      0.7       2.6      1.9    2.7   1.3     24.3     24.0       39        19        71        52        109        11    9.938       0.00          0    0.00
      L2    26/0        833   1.3      5.7     1.7      4.0       5.2      1.2    6.3   3.0     15.6     14.2       47       112       147        35        373        44    8.468       0.00          0    0.00
      L3    12/0        505   0.1      0.0     0.0      0.0       0.0      0.0    0.0   0.0      0.0      0.0        0         0         0         0          0         0    0.000       0.00          0    0.00
     Sum    47/7       1778   0.0      8.3     3.6      4.6      10.0      5.4    8.1   4.4     11.6     14.1       86       131       218        87        728       101    7.212       8.53          1 8526.25
     Int     0/0          0   0.0      2.4     0.8      1.6       2.7      1.2   11.5   6.1     12.0     13.6       20        43        63        20        203        23    8.845       0.00          0    0.00
    Flush(GB): accumulative 2.266, interval 0.444
    Stalls(secs): 0.000 level0_slowdown, 0.000 level0_numfiles, 8.526 memtable_compaction, 0.000 leveln_slowdown_soft, 0.000 leveln_slowdown_hard
    Stalls(count): 0 level0_slowdown, 0 level0_numfiles, 1 memtable_compaction, 0 leveln_slowdown_soft, 0 leveln_slowdown_hard

    ** DB Stats **
    Uptime(secs): 336.8 total, 60.4 interval
    Cumulative writes: 61584000 writes, 6480589 batches, 9.5 writes per batch, 1.39 GB user ingest
    Cumulative WAL: 0 writes, 0 syncs, 0.00 writes per sync, 0.00 GB written
    Interval writes: 11235257 writes, 1175050 batches, 9.6 writes per batch, 259.9 MB user ingest
    Interval WAL: 0 writes, 0 syncs, 0.00 writes per sync, 0.00 MB written

To see what happened, go here: https://github.com/facebook/rocksdb/blob/47b452cfcf9b1487d41f886a98bc0d6f95587e90/db/compaction_picker.cc#L430
* The for loop started with level 1, because it has the worst score.
* PickCompactionBySize on L429 returned nullptr because all files were being compacted
* ExpandWhileOverlapping(c) returned true (because that's what it does when it gets nullptr!?)
* for loop break-ed, never trying compactions for level 2 :( :(

This bug was present at least since January. I have no idea how we didn't find this sooner.

Test Plan:
Unit testing compaction picker is hard. I tested this by running my service and observing L0->L1 and L2->L3 compactions in parallel. However, for long-term, I opened the task #4968469. @yhchiang is currently refactoring CompactionPicker, hopefully the new version will be unit-testable ;)

Here's how my compactions look like after the patch:

    2014/08/22-08:50:02.166699 7f3400ffb700 ------- DUMPING STATS -------
    2014/08/22-08:50:02.166722 7f3400ffb700
    ** Compaction Stats [default] **
    Level   Files   Size(MB) Score Read(GB)  Rn(GB) Rnp1(GB) Write(GB) Wnew(GB) RW-Amp W-Amp Rd(MB/s) Wr(MB/s)  Rn(cnt) Rnp1(cnt) Wnp1(cnt) Wnew(cnt)  Comp(sec) Comp(cnt) Avg(sec) Stall(sec) Stall(cnt) Avg(ms)
    ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
      L0     8/5        404   1.5      0.0     0.0      0.0       4.3      4.3    0.0   0.0      0.0      9.6        0         0         0         0        463        88    5.260       0.00          0    0.00
      L1     2/2         60   0.9      4.8     3.9      0.8       4.7      3.9    2.4   1.2     23.9     23.6       80        23       131       108        204        19   10.747       0.00          0    0.00
      L2    23/3        697   1.0     11.6     3.5      8.1      10.9      2.8    6.4   3.1     17.7     16.6       95       242       317        75        669        92    7.268       0.00          0    0.00
      L3    58/14      2207   0.3      6.2     1.6      4.6       5.9      1.3    7.4   3.6     14.6     13.9       43       121       159        38        436        36   12.106       0.00          0    0.00
     Sum    91/24      3368   0.0     22.5     9.1     13.5      25.8     12.4   11.2   6.0     13.0     14.9      218       386       607       221       1772       235    7.538       0.00          0    0.00
     Int     0/0          0   0.0      3.2     0.9      2.3       3.6      1.3   15.3   8.0     12.4     13.7       24        66        89        23        266        27    9.838       0.00          0    0.00
    Flush(GB): accumulative 4.336, interval 0.444
    Stalls(secs): 0.000 level0_slowdown, 0.000 level0_numfiles, 0.000 memtable_compaction, 0.000 leveln_slowdown_soft, 0.000 leveln_slowdown_hard
    Stalls(count): 0 level0_slowdown, 0 level0_numfiles, 0 memtable_compaction, 0 leveln_slowdown_soft, 0 leveln_slowdown_hard

    ** DB Stats **
    Uptime(secs): 577.7 total, 60.1 interval
    Cumulative writes: 116960736 writes, 11966220 batches, 9.8 writes per batch, 2.64 GB user ingest
    Cumulative WAL: 0 writes, 0 syncs, 0.00 writes per sync, 0.00 GB written
    Interval writes: 11643735 writes, 1206136 batches, 9.7 writes per batch, 269.2 MB user ingest
    Interval WAL: 0 writes, 0 syncs, 0.00 writes per sync, 0.00 MB written

Yay for concurrent L0->L1 and L2->L3 compactions!

Reviewers: sdong, yhchiang, ljin

Reviewed By: yhchiang

Subscribers: yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D22305