Return MergeInProgress when fetching from transactions or WBWI with overwrite_key

Return MergeInProgress when fetching from transactions or WBWI with overwrite_key

Summary:
WriteBatchWithIndex::GetFromBatchAndDB only works correctly for overwrite_key=false.  Transactions use overwrite_key=true (since WriteBatchWithIndex::GetIteratorWithBase only works when overwrite_key=true).  So currently, Transactions could return incorrectly merged results when calling Get/GetForUpdate().

Until a permanent fix can be put in place, Transaction::Get[ForUpdate] and WriteBatchWithIndex::GetFromBatch[AndDB] will now return MergeInProgress if the most recent write to a key in the batch is a Merge.

Test Plan: more tests

Reviewers: sdong, yhchiang, rven, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D47817