Add enable_thread_tracking to DBOptions

Summary:
Add enable_thread_tracking to DBOptions to allow
tracking thread status related to the DB.  Default is off.

Test Plan:
export ROCKSDB_TESTS=ThreadList
./db_test

Reviewers: ljin, sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29289