[9.18] fix: usr: Apply the memory limit only to ADB database items

Resolver under heavy-load could exhaust the memory available for storing
the information in the Address Database (ADB) effectively evicting already
stored information in the ADB.  The memory used to retrieve and provide
information from the ADB is now not a subject of the same memory limits
that are applied for storing the information in the Address Database.

Closes #5127

Backport of MR !9954

Merge branch 'backport-5127-change-ADB-memory-split-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9976