Summary:
Clean up transactions to use the new RollbackToSavePoint api in WriteBatchWithIndex.

Note, this diff depends on Pessimistic Transactions diff and ManagedSnapshot diff (D40869 and D43293).

Test Plan: unit tests

Reviewers: rven, yhchiang, kradhakrishnan, spetrunia, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D43371