pool: Let permanent migration jobs react to access and sticky changes

Permanent migration jobs react to changes to the repository by adding or
removing files from the job queue. Until now migration jobs only picked up
state changes, ignoring changes to sticky flags and access time. This was
originally done to avoid that the pool to pool transfer issued by the migration
job would trigger the file to be added back into the queue. Since the original
implementation, we have however modified pool to pool transfers to no longer
trigger an atime update. Thus it is now feasible to let migration jobs react to
all changes.

The patch modifies the change notification to include both the old and the
updated information about the replica. The migration job only adds a replica
to the queue if the old information did not match while the new matches.

Target: trunk
Require-notes: yes
Require-book: yes
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6855/