[r=jtv][bug=1354511][author=jtv] Backport trunk r2668: When timing out nodes...

[r=jtv][bug=1354511][author=jtv] Backport trunk r2668: When timing out nodes that haven't finished commissioning, compare the last-update timestamp to the database clock (from which it was also generated), not to the Python-side clock. Timezone skew between the two could cause nodes to time out immediately, or hours late.