new: usr: Redesign the unreachable primaries cache

Previously, the cache for the unreachable primary servers was limited
to 10 entries (LRU) and a fixed 10 minutes delay for each entry, unless
removed forcibly by a new entry. The cache is now redesigned to remove the
10 entry limitation and to introduce delay values with exponential
backoff time - initially an unreachable primary server is cached as
being unreachable for 10 seconds, but each time the cache entry is expired
and the same server is added again during the eligibility period of the next
120 seconds, the delay time is doubled up until to the maximum of 640
seconds.

Closes #3992

Merge branch '3992-unreachable-cache-redesign' into 'main'

See merge request isc-projects/bind9!10393