Fix SimpleWriteTimeoutTest

Summary:
In column family's SanitizeOptions() [1], we make sure that min_write_buffer_number_to_merge is normal value. However, this test depended on the fact that setting min_write_buffer_number_to_merge to be bigger than max_write_buffer_number will cause a deadlock. I'm not sure how it worked before.

This diff fixes it by scheduling sleeping background task, which will actually block any attempts of flushing.

[1] https://github.com/facebook/rocksdb/blob/master/db/column_family.cc#L104

Test Plan: the test works now

Reviewers: yhchiang, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23103