Refactor StoreEvent, add MaybeRedactEvent, create an EventDatabase (#2989)

This PR changes the following:
- `StoreEvent` now only stores an event (and possibly prev event),
instead of also doing redactions
- Adds a `MaybeRedactEvent` (pulled out from `StoreEvent`), which should
be called after storing events
- a few other things

roomserver/internal/input/input_membership.go

@@ -53,7 +53,7 @@ func (r *Inputer) updateMemberships(
 	// Load the event JSON so we can look up the "membership" key.
 	// TODO: Maybe add a membership key to the events table so we can load that
 	// key without having to load the entire event JSON?
-	events, err := updater.Events(ctx, 0, eventNIDs)
+	events, err := updater.Events(ctx, nil, eventNIDs)
 	if err != nil {
 		return nil, err
 	}