NXCALS-980 - Delete snapshots that are older than X hours

@acc-logging-team - please have a look for approval.

Bug fix needed to keep snapshots for longer than just the last one. Due to an asynchronous nature of the underlying persistence provider it's rather difficult to express "the latest" therefore we stick to some time window. Later on this retention period should be configurable from the outside (see NXCALS-981).

This is rather urgent as it currently blocks develop