poolmanager: Avoid request leak

When a read pool request waits for a file to be replicated or staged, it
registers a callback for the reply from the pool.

We have observed a situation, in which there were many more entries in the hash
table of callbacks than there were requests. The only code path I could find that
could explain this behaviour is when resubmitting a stage request. I am not
certain when exactly this would happen, but it is the only scenario I can see
how entries in the map could leak.

The patch also removes the 'rc destroy' command. The implementation was incomplete
and dangerous to use.

Target: trunk
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7179/