Make activity count reading only happen in cron job.

If C* slows down for any reason, the memoized in-request get_counts can
run into lock contention and cause issues.  Instead, we'll just store
the activity on the event object in the cron job that does the counting
for socket broadcast purposes.
Apply a minor style tweak to activity broadcast code.
Record the activity of liveupdate events for later analysis.

This could allow for activity graphs etc.
update_activity: Don't crash entirely if any of the C* ops time out.