Summary:
Prototype of API to allow MyRocks to override default Mutex/CondVar used by transactions with their own implementations.  They would simply need to pass their own implementations of Mutex/CondVar to the templated TransactionDB::Open().

Default implementation of TransactionDBMutex/TransactionDBCondVar provided (but the code is not currently changed to use this).

Let me know if this API makes sense or if it should be changed

Test Plan: n/a

Reviewers: yhchiang, rven, igor, sdong, spetrunia

Reviewed By: spetrunia

Subscribers: maykov, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D43761