Summary:
Make use of the index in WriteBatchWithIndex to also count the number of sub-batches. This eliminates the need to separately scan the batch to count the number of sub-batches once a duplicate key is detected.
Closes https://github.com/facebook/rocksdb/pull/3529
Differential Revision: D7049947
Pulled By: maysamyabandeh
fbshipit-source-id: 81cbf12c4e662541c772c7265a8f91631e25c7cd
90eca1e616