Fix #258. benchmarkharness -- make bm_min_usec uint
Fix incorrect fixing of lint errors in ldb_cmd.cc

Summary: Fix incorrect fixing of lint errors in ldb_cmd.cc

Test Plan: reduce_levels_test

Reviewers: igor, sdong, ljin

Reviewed By: ljin

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28137
CompactionJob

Summary:
Long awaited CompactionJob class! Move most compaction-related things from DBImpl to CompactionJob, making CompactionJob easier to test and understand.

Currently this is just replicating exactly the same functionality with as little as change as possible. As future work, we should:
1. Add CompactionJob tests (I think I'll do that tomorrow)
2. Reduce CompactionJob's state that it inherits from DBImpl
3. Figure out how to do yielding to flush better. Currently I implemented a callback as we agreed yesterday, but I don't think it's a good long term solution.

This reduces db_impl.cc from 5000+ LOC to 3400!

Test Plan: make check, will add CompactionJob-specific tests, probably also move some tests from db_test to compaction_job_test

Reviewers: rven, yhchiang, sdong, ljin

Reviewed By: ljin

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D27957
Move VersionBuilder logic to a separate .cc file

Summary: Move all the logic of VersionBuilder to a separate .cc file

Test Plan: make all check

Reviewers: ljin, yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D28083