Introduce scan_statistic* tables and auto synch mechansim

[de-jcup]

Situation

While implementing/defining #19 we saw, that we cannot easily delete old scan_report table rows even when the data is no longer necessary.

Reason: We want to have those data still for statistics.

Wanted

1. Dedicated tables
   to can scan statistics which are automatically created and independent.

Ground design for one of these tables would be scan_report (without result and configuration).
TODO: define exactly which fields shall be inside these tables (+names of tables)

2. ScanAutoCleanupTriggerService introduced in Automatically remove old scans #19 shall then also delete the old scan_report table rows as well.
   Remark: The database query and it's tests were already implemented in Automatically remove old scans #19, but not called - see existing code for details.

Solution

TBD

[de-jcup]

With #2667 the last step is done, the auto cleanup does now work correctly. So closing this issue.