* add freezing and unfreezing of peers
* add rejoin packet
* revise handling of temporary invited connections
* rename "peer kill" packet to "peer leave" packet
* test rejoining in conference test
* use custom clock in conference test
This forces all the loop bodies to be executed at least once, which is
harmless since it just means one more tox event loop iteration. This
reduces the jitter we see in coverage measurements, which is partially
caused by loops sometimes being entered and sometimes not (because their
condition happens to randomly already be true).
