Summary:
1. Made `WriteBatchWithIndexInternal` into a class that stores the `DB*` or `DBOptions*`.
2. Changed the `GetFromBatch` method to be non-static and use an instance of the class. Added `MergeKey` methods to perform the merge itself and return any status.
This change unifies the multiple calls to the `MergeHelper` under a single wrapped API.
Closes https://github.com/facebook/rocksdb/issues/6683
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6851
Reviewed By: ajkr
Differential Revision: D21706574
Pulled By: pdillinger
fbshipit-source-id: 6860bd64d62669aaa591846e914eed3b674e68b1