DBTest.DynamicMemtableOptions to use single background compaction

Summary:
Now DBTest.DynamicMemtableOptions sets background compaction to be 4, without actually increasing thread pool size (even before the feature of automatic increasing it). To make sure the behavior stays the same after the automatic thread pool increasing, set it back to 1.

Hopefully it can fix the occasional failure of the test.

Test Plan: Run the test

Reviewers: igor, ljin

Reviewed By: ljin

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D28281