knock events sent over federation have no state but are not marked as outliers

[richvdh]

when we send out a knock event, we persist the sent event in the events table, but do not mark it out as an outlier.

We give it a state_group, but that state_group contains only the knock event itself.

[richvdh]

I'm not entirely sure what the effects of this will be, but it certainly isn't a good thing for the consistency of the room state and the DAG.