spacemanager: Use imported PoolMonitor to select link group

Space manager in some cases has to ask PoolManager to select a
link group. This patch changes the code to use an imported copy
of the PoolMonitor to achieve the same effect. This reduces
load on PoolManager and reduces latency in SpaceManager.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/6351/