Summary:
1. SetBackgroundThreads was not thread safe
2. queue_size_ does not seem necessary
3. moved condition signal after shared state change. Even though the original
   order is in practice ok (because the mutex is still held), it looks fishy
   and non-intuitive.

Test Plan: make check

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb, zshao

Differential Revision: https://reviews.facebook.net/D9825