Make global executors shutdown-safe

Make global executors shutdown-safe

Summary:
The `get*Executor()` APIs don't check whether the singletons
haven't been destroyed already. Add a check and allow to return
`nullptr` during shutdown.

Also do a general clean up of the code, there was no reason to use
three independent singletons (non-atomically destroyed) for each
executor.

Reviewed By: philippv, luciang

Differential Revision: D6589486

fbshipit-source-id: 20fb835db7e446bd811bbd6d5ddbc41db9e98b54