Remove tracking of time, only timeouts remain.

This makes multi-interface nodes less noisy.

We now accept unfeasible updates when the source hasn't changed, just don't
update the route's timestamp.  This makes it more likely that we have
up-to-date information while still avoiding loops.

Triggered updates are no longer sent when a route gets better.

This slows down convergence a little, but reduces the amount of traffic.

Rely on periodic updates instead.