Summary:
This allows an EventBase to exit loop() even if there are pending timeouts in the timer (make sure to cancelAll before deleting!)

Test Plan: Unit tests

Reviewed By: davejwatson@fb.com

Subscribers: doug, folly-diffs@, yfeldblum, chalfant

FB internal diff: D2111942

Signature: t1:2111942:1432934212:7ac1973bdbbfdbda714699ab373618399f29ef76