optimize key ID check when searching for matching keys

optimize key ID check when searching for matching keys

when searching a DNSKEY or KEY rrset for the key that matches
a particular algorithm and ID, it's a waste of time to convert
every key into a dst_key object; it's faster to compute the key
ID by checksumming the region, and then only do the full key
conversion once we know we've found the correct key.

this optimization was already in use in the validator, but it's
been refactored for code clarity, and is now also used in query.c
and message.c.