Expose a Magic File While Stratum1 Snapshotting is Running

This exposes a magic file called `.cvmfs_is_snapshotting` in the repository root while the snapshot procedure is running. Monitoring tools can use that to show nice spinner-animations while replication is in progress. Initially I put this status information in the mini-REST-API but it would be nice to have this information, even if the stratum 1 administrator decides to not expose the REST-API at all.