Query rooms with ACLs instead of all rooms (#3338)

This now should actually speed up startup times.
This is because _many_ rooms (like DMs) don't have room ACLs, this means
that we had around 95% pointless DB queries. (as queried on d.m.org)

roomserver/api/api.go

@@ -86,6 +86,9 @@ type RoomserverInternalAPI interface {
 		req *QueryAuthChainRequest,
 		res *QueryAuthChainResponse,
 	) error
+
+	// RoomsWithACLs returns all room IDs for rooms with ACLs
+	RoomsWithACLs(ctx context.Context) ([]string, error)
 }
 
 type UserRoomPrivateKeyCreator interface {