CA-137990: allow ring disconnection with pending requests

This patch series allows the ring to be disconnected even if there are pending requests. This allows the front-end to re-open the ring at will without having to take care of cleaning up its pending requests. This is especially important for the Windows front-end that receives the reset signal quite often when I/O is slow.  Although this behaviour seems to be the front-end's responsibility, we choose to fix this at the back-end, as it is far simpler to do so.

After a ring with pending requests has been disconnected, when these requests complete at the back-end with an error that will be ignored and such requests won't be retried.
Mostly for consistency with the way the log messages in this particular file. Using RING_XXX makes sense for the td-XXX.c files as it is used extensively there, using it here would result in the vast majority of calls being DPRINTF/EPRINTF and one or two being RING_XXX. Does this make sense?