Summary:
Although there are tests related to locking in transaction_test, this new test directly tests against TransactionLockMgr.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6599

Test Plan: make transaction_lock_mgr_test && ./transaction_lock_mgr_test

Reviewed By: lth

Differential Revision: D20673749

Pulled By: cheng-chang

fbshipit-source-id: 1fa4a13218e68d785f5a99924556751a8c5c0f31