Summary:
Support RollbackToSavePoint() in WriteBatch and WriteBatchWithIndex.  Support for partial transaction rollback is needed for MyRocks.

An alternate implementation of Transaction::RollbackToSavePoint() exists in D40869.  However, the other implementation is messier because it is implemented outside of WriteBatch.  This implementation is much cleaner and also exposes a potentially useful feature to WriteBatch.

Test Plan: Added unit tests

Reviewers: IslamAbdelRahman, kradhakrishnan, maykov, yoshinorim, hermanlee4, spetrunia, sdong, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D42723