read_only. It was set when a connection was tested, before the
connection was attached t the storage.  This made me wonder if the
flag and connection could get out of sync.  Because of details of the
complex connection dance, it appears that the flag will have a usable
value, almost by accident.  Ironically, if the storage was opened
read-only, this flag was set to true.  This all seemed very fragile,
and probably a bug magnet.  I refactored this so the flag is on the
connection, rather than the storage.  I also arranged that if the
storage is opened read-only, the flag is True.