Require runBeforeLoop callbacks to be canceled prior to EventBase destruction
Require runBeforeLoop callbacks to be canceled prior to EventBase destruction

Summary: This makes it similar to other other events registered with EventBase.

Reviewed By: yfeldblum

Differential Revision: D4331277

fbshipit-source-id: 5d728d4ae5faff1a72ed724b4e2a6c0dd6c3b02d

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