Reduce CernVM-FS Propagation Delay

New file system snapshots should be visible on the clients faster than currently. We aim for a ~5 mn delay from the point when the publishing finishes. Several points along the distribution chain should be tackled

• Speedup of the publishing process itself. This is largely completed by René's recent work.
• Notification service to start replication as part of the publish process (CVM-717).
• Caching and reuse of "object packs" developed as part of the distributed backend. This will drastically reduce the replication of small files.
• Stream change sets concurrently to authoritative storage and stratum 1s.
• Reduction (including validation) of the .cvmfspublished TTL. It can probably be reduced to values as low as 30 seconds.
• An interface to change the catalog default TTL. Possibly adjusting the default TTL from 15 minutes to 5 minutes.
• Investigate if the fuse-maintained kernel cache on clients can be actively purged to avoid the 1 minute delay to drain the caches.