Summary:
ColumnFamilyTest.FlushAndDropRaceCondition sometimes
hangs because the sync point, "FlushJob::InstallResults", sleeps
holding the DB mutex. Fixing it by releasing the mutex before sleeping.

Test Plan:
seq 1000 |parallel --gnu --eta 't=/dev/shm/rdb-{}; rm -rf $t;
mkdir $t && export TEST_TMPDIR=$t; ./column_family_test
-gtest_filter=*FlushAndDropRaceCondition* > $t/log-{}'

Reviewers: IslamAbdelRahman, anthony, kradhakrishnan, yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D53349