Lengthy delay when sending UDP to offline devices on a LAN

[PaulStoffregen]

When sending to an IP number on the same LAN which is offline, users report lengthy delays.

https://forum.pjrc.com/threads/30473-Timeout-delay-in-sendUDP-in-endPacket-when-target-is-not-online

[PaulStoffregen]

Pretty sure this only happens when sending to IP numbers in the same subnet (devices directly reachable on the local LAN) where we need to get a reply to an ARP query to find the 6 byte destination mac address.