spacemanager: Replace freespaceinbytes with availablespaceinbytes in link group table

Queries and triggers become simpler when we track available space rather
than free space.

On upgrade, the link group table will be updated. The table is small and the
schema change will be almost instantaneous. Note however that should dCache
be downgraded, the schema changes have to rolled back using one of the 'dcache
dabase rollback' commands.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6532/