net/arp: account for ARP delay, avoid duplicate packets on timeout

eth_rx() in the main reception loop may trigger sending a packet which
is already timed out (or will immediately) upon reception of an ARP reply.
As long as the ARP reply is pending, the timeout handler of a packet
should be postponed.
Happens on TFTP with bad network (e.g. WLAN).

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>