use a union for resp and qmin data

use a union for resp and qmin data

It's potentially confusing to use "resp_rdataset" for QNAME
minimization, but we can make it a union and have resp.rdataset
and qmin.rdataset using the same memory.

We can save even more space by using the same union to combine
qminname and resp_foundname and access them as qmin.name and
resp.foundname.