fix: usr: Improve the memory cleaning in the SERVFAIL cache

The SERVFAIL cache doesn't have a memory bound and the
cleaning of the old SERVFAIL cache entries was implemented
only in opportunistic manner.  Improve the memory cleaning
of the SERVFAIL cache to be more aggressive, so it doesn't
consume a lot of memory in the case the server encounters
many SERVFAILs at once.

Closes #5025

Merge branch '5025-improve-badcache-cleaning' into 'main'

See merge request isc-projects/bind9!9760