[Compaction] OnFlushCompleted() shows whether writes are slowed-down or stopped.  Allow BG compaction to be disabled.

Summary:
* OnFlushCompleted() shows whether writes are slowed-down or stopped.
* Allow BG compaction to be disabled.

Test Plan:
Add DisableBGCompaction test in listener_test
make listener_test -j32
./listener_test

Reviewers: sdong, ljin, igor, dhruba

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21885