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