Summary: In some db_test tests sync points are not cleared which will cause unexpected results in the next tests. Clean them up in test cleaning up.

Test Plan:
Run the same tests that used to fail:

build using USE_CLANG=1 and run
./db_test --gtest_filter="DBTest.CompressLevelCompaction:*DBTestUniversalCompactionParallel*"

Reviewers: rven, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D36429