Fix EventBaseLoopController destruction races

Fix EventBaseLoopController destruction races

Summary:
Existing scheduleThreadSafe implementation had 2 potential races on destruction:
1. (very unlikely) insertHead is complete, but fiber loop is already running on another thread, so it finishes processing all of the fibers, destroys FiberManager or EventBase or both. By the time we get to scheduleThreadSafe EventBaseLoopController is already destoyed
2. (more likely) scheduleThreadSafe is complete, but FiberManager loop which is already running, picks complete fiber, finishes the processing. After that FiberManager may be destoyed. So when EventBase actually executes the callback FiberManager is already dead.

This solution fixes both races. Holding the alive shared_ptr when completing sheduleThreadSafe assures EventBase can't be destoyed until its completed (or it won't try to schedule anything after EventBase was destroyed). Locking alive weak_ptr in the EventBase loop callback ensures FiberManager and thus EventBaseLoopController were not destroyed yet (they can be destoyed only by the same thread which is running EventBase loop).

Reviewed By: spalamarchuk

Differential Revision: D2763206

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