Implement incremental hashing

When computing the hash for the hashtables (both isc_ht and isc_hashmap), we sometimes use dns_name_t + something (something could be class, type or arbitrary value).

Currently, this requires copying the content of dns_name_t into packed structure with the "something" appended, so we can hash that as a single uint8_t array.

Incremental rehashing would allow "fractured" keys.