Make watch rules use revisions when watching Status.

Adjust the _WatchHelper class (and it's RuleHelper base) to track and use
status revisions when getting a Status deferred to watch for rule changes.
When Status is updated, notifiy it's deferreds right away.

We used to wait until the next round of the reactor to notify deferreds of
Status updates. Now do it right away. Mostly, I hope this will make some
testing simpler.

Tweak web_resources to handle gettins notifications right away since it
could break.