Summary:
Add stats for read/write bytes and input/output file of internal operations and break down by operation type (being flush, compaction or GC). Later after we introduce vtable we can further breakdown compaction into top levels compaction, L_n-1 compaction and Ln compaction.

Test Plan:
Run db_bench and get sample output:
```
2019/08/29-23:49:29.593805 7f6fd97fa700 (Original Log Time 2019/08/29-23:49:29.593742) Titan internal stats for column family [default]:
2019/08/29-23:49:29.593813 7f6fd97fa700 (Original Log Time 2019/08/29-23:49:29.593753) OP          COUNT  READ(GB)  WRITE(GB) IO_READ(GB) IO_WRITE(GB)  FILE_IN FILE_OUT
2019/08/29-23:49:29.593821 7f6fd97fa700 (Original Log Time 2019/08/29-23:49:29.593760) ---------------------------------------------------------------------------------
2019/08/29-23:49:29.593830 7f6fd97fa700 (Original Log Time 2019/08/29-23:49:29.593768) Flush          3        0.0       34.7         0.0         18.0        0        3
2019/08/29-23:49:29.593839 7f6fd97fa700 (Original Log Time 2019/08/29-23:49:29.593784) Compaction     1        0.0        0.0         0.0          0.0        0        0
2019/08/29-23:49:29.593848 7f6fd97fa700 (Original Log Time 2019/08/29-23:49:29.593793) GC             1      360.9        1.3       354.3          2.8        8        0
```