Summary:
The `TransactionTest.MultiGetBatchedTest` were failing with unprepared batches because we were not using the correct callbacks. Override MultiGet to pass down the correct ReadCallback. A similar problem is also fixed in WritePrepared.
This PR also fixes an issue similar to (https://github.com/facebook/rocksdb/pull/5147), but for MultiGet instead of Get.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5634
Differential Revision: D16552674
Pulled By: lth
fbshipit-source-id: 736eaf8e919c6b13d5f5655b1c0d36b57ad04804