fix: usr: Restore delegdb size after `rndc flush`

When the delegation database was flushed using `rndc flush`, its size was also reset but not restored. As a result, after `rndc flush` was used at least once, the delegation database size could grow unbounded. This has now been fixed.

Merge branch 'colin/delegdb-config' into 'main'

See merge request isc-projects/bind9!12101