Summary:
Currently AddPrepared is performed only on the first sub-batch if there are duplicate keys in the write batch. This could cause a problem if the transaction takes too long to commit and the seq number of the first sub-patch moved to old_prepared_ but not the seq of the later ones. The patch fixes this by calling AddPrepared for all sub-patches.
Closes https://github.com/facebook/rocksdb/pull/3651

Differential Revision: D7388635

Pulled By: maysamyabandeh

fbshipit-source-id: 0ccd80c150d9bc42fe955e49ddb9d7ca353067b4