Need support for cleaning up pool of no-longer-referenced packages

[Paul Goins]

We've encountered at least one environment where the apt-mirror unit grew in size until it ran out of disk.

While the charm has an option for deleting snapshots, all it does is delete the "metadata", i.e. the snapshotted "dists" directory of the mirrored repositories. It doesn't clean up the packages in the shared pool.

We need to have some sort of script or cleanup action which allows us to reclaim disk space from no-longer-used packages. It would need to clean up files in the pool which no longer have any references from any of the current snapshots.