Make sure there are no outstanding timers in the destructor

Make sure there are no outstanding timers in the destructor

Summary: This is the solution for our `HHWheelTimer` crashes, as suggested in D2617966#25.

djwatson also mentioned that maybe there should be some logging in the destructor if there are outstanding callbacks. I couldn't think of anything that would add to the `assert` that already exists in `destroy`. I'm open to suggestions though.

Reviewed By: djwatson

Differential Revision: D2628154

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