Ensure timers/sessions added to `BatonASIO` are cancelable

Adding a timer/session to a BatonASIO while it is polling translates to a task scheduled on the baton. The task will add the timer/session to the baton once it's done polling.

This behavior has an unwanted consequence: if we attempt to cancel the timer/session before the scheduled task is processed, we cannot find the timer/session on the baton. The current implementation of BatonASIO misinterprets that for an already canceled timer/session and simply returns from cancelTimer and cancelSession.

The expected outcome for this ticket is to fix this behavior so that timers/sessions added to BatonASIO while it is polling are also cancelable.

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There are two unit-tests the exercise the conditions described in this ticket:

• AddAndRemoveSessionWhileInPoll
• AddAndRemoveTimerWhileInPoll