Fix two problems with dns.xfr.make_query():

Fix two problems with dns.xfr.make_query():

1) We always used class IN instead of using the class of the txn manager.
2) We directly appended to the authority section instead of using
   find_rrset(), which meant that our changes were not indexed and
   would break if other code tried to use find_rrset() to find what
   we added.