polypath asymmetric routing: Handle data packet dupes

polypath asymmetric routing: Handle data packet dupes

If polypath has asymmetric routing, where path A upload is faster than
path B, but path A download is completely broken, then we need to
retain both paths A and B in our list of transport peers.

Stepping back, we need to treat dupes of recent packets as evidence
that the peer is at that address, even though we do not pass the
packets to the netlink.

We don't want to do this for arbitrarily old data packets.  The
heuristic we use here is rather crude: packets which are more than
32 (by default) out of order are treated as too old.  This will be too
short if path A is a high-bandwidth link and path B is quite slow, and
it will be too long if the link is very idle.  Hopefully this will not
matter in practice.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>