Summary:
The MemTableRep API was broken by this commit: 813719e9
This patch reverts the changes and instead adds InsertKey (and etc.) overloads to extend the MemTableRep API without breaking the existing classes that inherit from it.
Closes https://github.com/facebook/rocksdb/pull/3513

Differential Revision: D7004134

Pulled By: maysamyabandeh

fbshipit-source-id: e568d91fe1e17dd76c0c1f6c7dd51a18633b1c4f