Reschedule circuit timeouts when the configuration changes

This is a followup task from !169 (merged): when the circuit manager's configuration changes or network parameters, we should consider adjust the timeout values of all the open circuits.

For efficiency, it might be better if, instead of launching a new timeout task unconditionally, we just adjust the timing of the old timeout task. That will require a little new infrastructure: we should think about this.

(In other words, let's get the design right first before we move ahead and build it.)