Fix leaking timer goroutines

[andrewdeandrade]

This fixes two leaking goroutines from the ringpop timers. There is still one more leaking goroutine that I'm aware of in rcrowley/go-metrics:

https://github.com/rcrowley/go-metrics/blob/master/meter.go#L218-L225

That's a ticker that would also require a stop channel.

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