Minor compaction logging improvements
1) make summary less likely to be truncated
2) format human-readable file sizes in summary
3) log the motivation for each universal compaction
An initial implementation of kCompactionStopStyleSimilarSize for universal compaction
address code review comments on 5e3aeb5f8e
- reduce string copying in Compaction::Summary
- simplify file number checking in UniversalCompactionStopStyleSimilarSize unit test
Merge pull request #47 from mlin/kCompactionStopStyleSimilarSize

An initial implementation of kCompactionStopStyleSimilarSize for universal compaction