pool: Manage periodic health check through Spring

Minor refactoring prior to adding new features to the periodic
health check.

Note that the output of the 'info' command of a pool has changed
as it no longer shows the 'Check Repository  : true' line.

Target: trunk
Require-notes: no
Require-book: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5772/