spacemanager: Add automatic and manual purge of perished spaces

Space manager kept spaces indefinitely, even after they had been released or
expired. Due to implicit space reservation, these reservation would add up over
time.  This patch adds automatic deletion of released or expired (perished)
reservations once a certain time has passed since the expiration date. Empty
reservations are purged as soon as they are marked as expired or released.

The patch also adds the 'purge spaces' admin command to purge all perished
spaces immediately, even those that don't have an expiration date. No files are
deleted from dCache and no spaces that are not already released or expired are
purged.

The patch moves the related business logic out of the data access layer. The
fluent API is extended to cope with the new requirements.

Introduces these new properties:

spacemanager.perished-space-purge-delay
spacemanager.perished-space-purge-delay.unit

Introduces this new admin ommand:

purge spaces

Target: trunk
Require-notes: yes
Require-book: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6461/