Draft: Switch the glue table implementation to isc_hashmap

Ondřej Surý requested to merge 4045-use-isc_hashmap-for-glue_table into main

The glue table inside dns_rbtdb was using custom hashtable, with full table rehashing (etc.). Instead of having yet another hash table implementation, refactor the glue table to use isc_hashmap and use the upgrade rwlock mechanics to make the find-add mechanics more efficient.

Closes #4045 (closed)

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