chg: dev: Use lock-free hashtable for storing resolver fetch contexts

Replace the locked hashmap with the lock-free hashtable from the RCU
library and protect the fetch contexts against reuse by replacing the
libisc reference counting with urcu_ref that can soft-fail in situation
where the reference count is already zero.  This allows us to easily
skip re-using the fetch context if it is already in process of being
destroyed.

Merge branch 'ondrej/use-urcu-lfht-for-resolver-tables' into 'main'

See merge request isc-projects/bind9!10653