Summary:
Removes the need to store keys in incremental order by sorting them after before inserting into MapBuffer.
Updates MapBuffer to support random access on C++ side, actually retrieving values by keys and not bucket index.
Changelog: [Internal]
Reviewed By: mdvacca
Differential Revision: D33611758
fbshipit-source-id: c81e970613c8ecc688bfacb29ba038bf081a0c0f