change ForEachRoom to use read-only cursors by default. it can be overridden to

change ForEachRoom to use read-only cursors by default. it can be overridden to
still use read/write cursors by doing:

 cdb_begin_transaction();
 ForEachRoom(...);
 cdb_end_transaction();

the only place I found where it appears necessary to do so is check_ref_counts,
so this checkin affects that function too.