Fix destruction race-condition in ThreadWheelTimekeeper
Fix destruction race-condition in ThreadWheelTimekeeper

Summary: It was possible for interruptHandler() to be called concurrently with the ThreadWheelTimekeeper shutdown resulting in use-after-free when trying to schedule work on the EventBase.

Reviewed By: yfeldblum

Differential Revision: D9948555

fbshipit-source-id: 4eff3cce0488778559dffc47bfc6eb59a65bb203

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly