[GL #3709] reordered the dns_rdataset_disassociate call to after
the dns_resolver_createfetch call resulting in qctx->nsrrset still
being associated when dns_resolver_createfetch is called in
resume_dslookup (7e4e125e). Revert that part of the change and add
comments as to why the multiple dns_rdataset_disassociate calls are
where they are.
5ff55e13e8