Resolve "Database lease backend statistics contain statistics for non-existent subnets"
requested to merge 2033-database-lease-backend-statistics-contain-statistics-for-non-existent-subnets into master
Closes #2033 (closed)
This MR makes sure the stats returned are correct and only for known subnets. In addition it logs if any stats for non-existent subnets were detected along the way. It does not attempt to clean up DB content.