avoid stalled source

A source given up for not answering our sync messages could become the
source again straight away.  Whoever asks first after the reset takes the
role, and that's often the same peer, which would then stall for another
delivery limit.

Now, a stalled source isn't chosen again for one more delivery_limit.
If it's the only candidate, catching up waits that long and then tries
it again.  Before, a retry with that peer couldn't have helped any
sooner anyway: a source is only given up after a whole delivery limit
without a sync message, and that clock restarts when it is chosen again.

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