Fix race in timer stoppage

[StephenButtolph]

Why this should be merged

It is possible for:

1. Stop to return false
2. have the select statement run before the event is pushed to the timer.C channel.
3. Have the select statement run the default case
4. Have the timer then push the event onto the timer.C channel
5. The timerPool will then include a timer that has already fired (breaking the documented invariant)

How this works

Rather than draining the channel in the defer (which doesn't have enough information to safely drain the channel) - the channel is drained in the for loop.

How this was tested

Open to feedback on how to test this, It seems like any test for this would be inherently racy... But open to suggestions.