Prune Snaphsots after Sync

[rsoika]

Snapshots should be deleted after a snapshot was successfully archived into the Cassandra cluster.

For this feature we move the sync service directly into the workflow instance but without adding the Cassandra libs. This can be easily realized because we already have the rest api implemented.

Solution:

• After each Scnapshot creation a EventLog entry is created.
• An async timer service pushes the snapshots into the archive
• After successull push the snapshot and eventLog entry can be deleted (in the first step just a soft delete)
• The SnapshotArchive Service will be changed and will no longer permanently sync but provide a 'ReSync' method.