DHCP maintenance

[Faeranne]

DHCP renew needs to be run to keep from re-assigning the ip to other devices.
The Arduino Ethernet library handles this with Ethernet.maintain(), which should be run periodically, preferably every loop, but no less than once every 30 seconds.

[Faeranne]

https://www.arduino.cc/en/Tutorial/DhcpAddressPrinter gives an example of using maintain()