Infinite loop in is_any_outdated()

[bachdavi]

is_any_outdated() uses an internal timer which advance_domain() changes.
If we probe a Dataflow that has not started or has no current inputs we might run into an infinite loop in the server at : worker.step_while(|| server.is_any_outdated());
because we already advanced the inputs but the probe still only sees the "old" timestamps.

[comnik]

This should also be addressed by f09ec28, again, tests required.

[comnik]

Impossible to have an infinite loop, because we step fixed amounts now :)