Keep around the old (buggy) version of the prune_event function so that we can use it to check signatures for events on old servers

Try to ensure we don't persist an event we have already persisted. In persist_event check if we already have the event, if so then update instead of replacing so that we don't cause a bump of the stream_ordering.

Don't fail an entire request if one of the returned events fails a signature check. If an event does fail a signature check, look in the local database and request it from the originator.

Rejections

Client v2 sync

Add doc string for __nonzero__ overrides for sync results, raise not implemented if the client attempts to do a gapless sync

Conflicts:
	synapse/storage/schema/im.sql

Replication split

Means that now you can't do exact matches even in override rules,
but I think we can live with that. Advantage is that you'll now
always get back what was put in to the API.

Rejections storage