BUG/MAJOR: cli: show sess <id> may randomly corrupt the back-ref list

BUG/MAJOR: cli: show sess <id> may randomly corrupt the back-ref list

show sess <id> puts a backref into the session it's dumping. If the output
is interrupted, the backref cannot always be removed because it's only done
in the I/O handler. This can randomly corrupt the backref list when the
session closes, because it passes the pointer to the next session which
itself might be watched.

The case is hard to reproduce (hundreds of attempts) but monitoring systems
might encounter it frequently.

Thus we have to add a release handler which does the cleanup even when the
I/O handler is not called.

This issue should also be present in 1.4 so the patch should be backported.