chg: dev: Simplify the delegation database memory management

This is an internal simplification of the delegation database's memory
management, replacing the per-thread eviction lists and deferred,
cross-thread record cleanup with a single shared eviction list and
immediate cleanup. There is no change to how delegations are cached or
resolved.

Merge branch 'ondrej/delegdb-shared-sieve-lru' into 'main'

See merge request isc-projects/bind9!12181