Fix hang with large write batches and column families.

Fix hang with large write batches and column families.

Summary:
This diff fixes a hang reported by a Github user.
https://www.facebook.com/l.php?u=https%3A%2F%2Fgithub.com%2Ffacebook%2Frocksdb%2Fissues%2F595%23issuecomment-96983273&h=9AQFYOWlo
Multiple large write batches with column families cause a hang.
The issue was caused by not doing flushes/compaction when the
write controller was stopped.

Test Plan: Create a DBTest from the user's test case

Reviewers: igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D37929