Fix WAL synced

Summary: Uhm...

Test Plan: nope

Reviewers: sdong, yhchiang, tnovak, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23343
WriteThread

Summary: This diff just moves the write thread control out of the DBImpl. I will need this as I will control column family data concurrency by only accessing some data in the write thread. That way, we won't have to lock our accesses to column family hash table (mappings from IDs to CFDs).

Test Plan: make check

Reviewers: sdong, yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23301
standardize scripts to run RocksDB benchmarks

Summary:
Hope these scripts will allow people to run/repro benchmark easily
I think it is time to re-run flash benchmarks and report results
Please comment if any other benchmark runs are needed

Test Plan: ran it

Reviewers: yhchiang, igor, sdong

Reviewed By: igor

Subscribers: dhruba, MarkCallaghan, leveldb

Differential Revision: https://reviews.facebook.net/D23139
Fix #284