srm: Schedule resubmit of pin request to avoid deadlock

Since the callback happens on the delivery thread, we shouldn't do
anything heavy in it (such as doing a protocol support check that
may involve querying login broker - something that would cause a
deadlock as the reply would not be deliverable).

Target: trunk
Request: 2.9
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6978/