spacemanager: Reduce latency for some pool selection messages

Some pool selection messages do not require any processing in
space manager and can be forwarded as is. This patch changes
the logic to bypass the additional queueing in those cases.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Acked-by: Albert Rossir <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/6353/