srm: Fix deadlock in pinning

Another case of me being too optimistic about how much or how little
happens in a message callback. In this case pinning uses the remote
pool monitor, which may tricker a fetch from pool manager.

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/6979/