Summary:
The background threads are necessary for compaction.
For slower storage, it might be necessary to have more than
one compaction thread per DB. This patch allows creating
a configurable number of worker threads.
The default reamins at 1 (to maintain backward compatibility).

Test Plan:
run all unit tests. changes to db-bench coming in
a separate patch.

Reviewers: heyongqiang

Reviewed By: heyongqiang

CC: MarkCallaghan

Differential Revision: https://reviews.facebook.net/D5559