Handle the fact that events can be persisted out of order, and so to get
the "current max" stream token becomes non trivial - as we need to make
sure that *all* stream tokens less than the current max have also
successfully been persisted.

Hotfixes v0.8.1 r3

Don't yield on notifying all listeners

Fix a notifier leak

Add paranoia checks to make sure that we evict stale NotificationListeners when we are about to process them

Fix bug where we didn't inform the NotificataionListeners about new rooms they have been subscribed to. This meant that the listeners didn't clean themselves up fully from all the dicts

Caused by a bad if check, which incorrectly executes for both 0 and None,
when None was the original intent.