- the goal of this change is for AAAA filtering to be fully contained
   in the query logic, and implemented at discrete points that can be
   replaced with hook callouts later on.
 - the new code may be slightly less efficient than the old filter-aaaa
   implementation, but maximum efficiency was never a priority for AAAA
   filtering anyway.
 - we now use the rdataset RENDERED attribute to indicate that an AAAA
   rdataset should not be included when rendering the message. (this
   flag was originally meant to indicate that an rdataset has already
   been rendered and should not be repeated, but it can also be used to
   prevent rendering in the first place.)
 - the DNS_MESSAGERENDER_FILTER_AAAA, NS_CLIENTATTR_FILTER_AAAA,
   and DNS_RDATASETGLUE_FILTERAAAA flags are all now unnecessary and
   have been removed.