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.
Will we end up recording the stats twice? We haven't removed the original block in line 397 (in the new file)