Summary:
Unrevert the previous fix to propagate error status, and an additional fix to not treat a memtable lookup MergeInProgress status as an error.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6403
Test Plan:
Unit tests
Tried running stress tests but couldn't repro the stress failure
Differential Revision: D19846721
Pulled By: anand1976
fbshipit-source-id: 7db10cccbdc863d9b559497f0a46b608d2488ca4