[9.20] chg: usr: Fix CPU spikes and slow queries when cache approaches memory limit

When the cache grew close to the configured max-cache-size, every
subsequent entry triggered all worker threads to run cache cleanup at
once, causing CPU spikes and a drop in query throughput. Cleanup is now
spread probabilistically across inserts as memory approaches the limit,
so the work is distributed evenly instead of piling up at the threshold.

Backport of MR !1002

Merge branch '5891-improve-overmem-cleaning-9.20' into 'security-bind-9.20'

See merge request isc-private/bind9!1000