Summary:
We want applying deltas to be an atomic operation, from incrementing the delta message ID to updating the relevant maps.
This is a simple approach to synchronize the corrsponding method.
We will probably need to go with a more sophisticated approach, that makes sure that deltas are applied in order. That would also allow us to lock only on writes.
Reviewed By: kathryngray
Differential Revision: D6846560
fbshipit-source-id: 175a80b4e39223883e397d75e20109fc12a2a878