Delay binding glue to rdataset

Delay binding glue to rdataset

The dns_glue struct currently contains four dns_rdataset structs to hold
the glue. These structs are over 100 bytes each because they need to be
able to hold data for multiple types of databases.

Since the dns_glue_t type is only used by qpzone, we can instead hold
pointers to the vecheaders directly, and only bind the vecheaders to
the rdatasets when adding the glue to the message.