Summary:
This PR wants to improve support for transaction in C-API:
* Support two-phase commit.
* Support `get_pinned` and `multi_get` in transaction.
* Add `rocksdb_transactiondb_flush`
* Support get writebatch from transaction and rebuild transaction from writebatch.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9252

Reviewed By: jay-zhuang

Differential Revision: D36459007

Pulled By: riversand963

fbshipit-source-id: 47371d527be821c496353a7fe2fd18d628069a98