dns: handle multiple replies for single requests

dns: handle multiple replies for single requests

The world is a mess. A single DNS request may yield multiple, subtly
different responses; all with the same DNS protocol-level ID.

Last response wins in terms of what ends up in the cache.

First response wins in terms of the response sent to the client. Hard
to do otherwise since we have no clue that more than one answer will
be forthcoming.

Type: fix
Ticket: VPP-1749
Signed-off-by: Dave Barach <[email protected]>
Change-Id: I3175a40eb1fea237048d16b852a430f5ab51eaef
(cherry picked from commit e8d2dcb6a619f0884ece2284a286f21b3aa77e5a)