Refactor StoreEvent and create a new RoomDatabase interface (#2985)

This PR changes a few things:
- It pulls out the creation of several NIDs from the `StoreEvent`
function to make the functions more reusable
- Uses more caching when using those NIDs to avoid DB round trips

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, eventNIDs)
+	events, err := updater.Events(ctx, 0, eventNIDs)
 	if err != nil {
 		return nil, err
 	}