Few changes here:

• No need to handle duplicate keys in RocksUniqueCursor. Patch is mostly port from WiredTiger: https://github.com/mongodb/mongo/commit/3d5aee54e4f45cc516d64bfef0d682e93e264258#diff-8cc7939b6e3618bb7b6a7fc74ef331e7R823
• Cache repeated calls to locate(), which hides RocksDB's bad performance in presence of lots of deletes. This makes aggregation test pass.