Summary: We should not be calling InternalStats methods outside of the mutex.

Test Plan:
COMPILE_WITH_TSAN=1 m db_test && ROCKSDB_TESTS=CompactionTrigger ./db_test

failing before the diff, works now

Reviewers: yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D32127