Mark old events as outliers.
Mark old events as outliers.

This is to fix the issue where if a remote server sends an event
that references a really "old" event, then the local server will pull
that in and send to all clients.

We decide if an event is old if its depth is less than the minimum depth
of the room.

matrix-org/synapse,rzr/synapse,illicitonion/synapse,matrix-org/synapse,illicitonion/synapse,illicitonion/synapse,howethomas/synapse,iot-factory/synapse,rzr/synapse,TribeMedia/synapse,illicitonion/synapse,matrix-org/synapse,TribeMedia/synapse,iot-factory/synapse,iot-factory/synapse,howethomas/synapse,TribeMedia/synapse,illicitonion/synapse,matrix-org/synapse,rzr/synapse,TribeMedia/synapse,rzr/synapse,iot-factory/synapse,rzr/synapse,matrix-org/synapse,howethomas/synapse,TribeMedia/synapse,iot-factory/synapse,howethomas/synapse,matrix-org/synapse,howethomas/synapse