Print MB per second compaction throughput separately for reads and writes

Summary:
From this line there used to be one column (MB/sec) that includes reads and writes. This change splits it and for real workloads the rd and wr rates might not match when keys are dropped.
2014/09/29-17:31:01.213162 7f929fbff700 (Original Log Time 2014/09/29-17:31:01.180025) [default] compacted to: files[2 5 0 0 0 0 0], MB/sec: 14.0 rd, 14.0 wr, level 1, files in(4, 0) out(5) MB in(8.5, 0.0) out(8.5), read-write-amplify(2.0) write-amplify(1.0) OK

Test Plan:
make check, grepped LOG

- begin *PUBLIC* platform impact section -
Bugzilla: #
- end platform impact -

Reviewers: igor

Differential Revision: https://reviews.facebook.net/D24237