Summary:
Why do we need a dedicated registry class?
* We need to simplify registry-related logic in Scheduler.
* We need to couple threading aspect of the registry with the registry itself, otherwise it's not clear why exactly we acquire the mutex. We also should not acquire the mutex in a per-method way (as we did before), because it's incorrect and misleading (only lines that access the registry should by protected).
* We need to have a way to share the registry with other classes (e.g. UIManager) without passing a reference to the whole Scheduler.
Reviewed By: mdvacca
Differential Revision: D13036550
fbshipit-source-id: 644da910e823666c586834a3da2b4cdcb90eebb2
