Move commonly used dns_rdataset_t structures to the verification context structure

Move commonly used dns_rdataset_t structures to the verification context structure

Eight structures representing four RRsets and their signatures are
commonly accessed throughout dns_zoneverify_dnssec().  Move them into
the structure representing a verification context.  While this does not
really simplify currently existing code, it will facilitate passing data
around between smaller functions that dns_zoneverify_dnssec() is about
to get split into.