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/acls/acls_test.go

@@ -116,7 +116,7 @@ var (
 
 type dummyACLDB struct{}
 
-func (d dummyACLDB) GetKnownRooms(ctx context.Context) ([]string, error) {
+func (d dummyACLDB) RoomsWithACLs(ctx context.Context) ([]string, error) {
 	return []string{"1", "2"}, nil
 }