[9.18] fix: usr: Improve the resolver performance under attack

[9.18] fix: usr: Improve the resolver performance under attack

A remote client can force the DNS resolver component to consume the memory faster than cleaning up the resources for the canceled resolver fetches due to `recursive-clients` limit. If the such traffic pattern is sustained for a long period of time, the DNS server might eventually run out of the available memory. This has been fixed.

It should be noted that when under such heavy attack for BIND 9 version both with and without the fix, no outgoing DNS queries will be successful as the generated traffic pattern will consume all the available slots for the recursive clients.

Merge branch '5110-backport-the-hashtable-use-for-fetchcontexts-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9961