srm: Avoid duplicate status code update on status query

Container requests calculate their status code based on the status of
their file requests: However once they reached a final state, that
final state should be used rather than update the state several times.

When requests expire, I have observed illegal state transition errors
caused by trying to update the state several times.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/7000/