Summary:
This implements a view onto an EventBase, which can keep track of all tasks created through it and join them on destruction. Multiple VirtualEventBases can be backed by the same EventBase. This can be useful to have the same IO thread/thread-pool be shared between multiple libraries, allowing them to each have it's own VirtualEventBase and control its lifetime.

Since VirtualEventBase also supports LoopKeepAlive and onDestruction functionality, it can be easily integrated with FiberManagerMap.

Reviewed By: yfeldblum

Differential Revision: D4158719

fbshipit-source-id: 7df964f36e5276e2b5665fd8394ea2e187aa298c