Track callback life cycle for HHWheelTimer and EventBase

Track callback life cycle for HHWheelTimer and EventBase

Summary: Further race is presented in HHWheelTimer and EventBase, since changing them to track callback life cycle is more involving, I am hacking around it to prove the concept. With the *fix*, no reproduce of the segmentation fault anymore.

This is for proving the concept, code will be cleaned up if we agree this is reasonable direction to pursue.

Reviewed By: fugalh

Differential Revision: D2577715

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