[9.18] fix: usr: Clean enough memory when adding new ADB names/entries under memory...

The ADB memory cleaning is opportunistic even when we are under
memory pressure (in the overmem condition).  Split the opportunistic
LRU cleaning and overmem cleaning and make the overmem cleaning
always cleanup double of the newly allocated adbname/adbentry to
ensure we never allocate more memory than the assigned limit.

Backport of MR !10637

Merge branch 'backport-ondrej/enforce-memory-cleanup-in-ADB-when-overmem-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10708