Summary:
In order to better understand compaction done by different priority thread pool, we now collect compaction stats by priority and also print them to info LOG through stats dump.

```
** Compaction Stats [default] **
Priority    Files   Size     Score Read(GB)  Rn(GB) Rnp1(GB) Write(GB) Wnew(GB) Moved(GB) W-Amp Rd(MB/s) Wr(MB/s) Comp(sec) CompMergeCPU(sec) Comp(cnt) Avg(sec) KeyIn KeyDrop
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Low      0/0    0.00 KB   0.0     16.8    11.3      5.5       5.6      0.1       0.0   0.0    406.4    136.1     42.24             34.96        45    0.939     13M  8865K
High      0/0    0.00 KB   0.0      0.0     0.0      0.0      11.4     11.4       0.0   0.0      0.0     76.2    153.00             35.74     12185    0.013       0      0
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5050

Differential Revision: D14408583

Pulled By: miasantreble

fbshipit-source-id: e53746586ea27cb8abc9fec35805bd80ed30f608