Previously we had an issue that every other update_active_status
request for a particular realm would result in doing the expensive
query to compute the list of active users in that realm. It turned
out this was because on every update_active_status request, we'd queue
an event that would have the effect of clearing the cache, even if
nobody's tatus changed. This fixes that issue, by only clearing the
cache for a realm if someone's status actually changed (or the 60s
timeout expires).
(imported from commit d5b829fe255a31c8cecb58458738f1e72a2cf6de)