Split EventBaseThread from ScopedEventBaseThread
Summary:
[Folly] Split `EventBaseThread` from `ScopedEventBaseThread`.
Now `ScopedEventBaseThread` is really scoped and immovable, while `EventBaseThread` is movable and can be started and stopped.
Users which will never move, and will never start or stop, the `ScopedEventBaseThread` can continue using it. Users which need to move, or which need to start and stop, the object will use `EventBaseThread` instead.
Reviewed By: andriigrynenko
Differential Revision:
D4338447
fbshipit-source-id:
57c186630bc199a7a7b7223b1fcb077ce3d86743