spacemanager: Split selectPool method

The selectPool method had to nearly unoverlapping flows. This
patch splits the method into two methods, one for the request
flow and one for the reply flow.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6332/