Add write-through behavior to ChangesStore

Add write-through behavior to ChangesStore

Summary:
When the changes stack is empty, write changes through to the underlying fallback store.

This lets us model the local changes behavior for the naming table in hh_server: for writes, when the stack is empty, we write to sharedmem. For reads, we read from the stack, then fallback to sharedmem, then fallback to the SQLite DB. If ChangesStore supports write-through behavior, then we can model that behavior with a ChangesStore wrapping a DeltaStore, where the delta is a sharedmem Store and the DeltaStore's fallback is the SQLite DB. This diff refactors hackrs_provider_backend::naming_table to work this way.

Reviewed By: shayne-fletcher

Differential Revision: D36465267

fbshipit-source-id: 4bba55b87a7d314f74b253ef0a17496f9080e750