Send a keep alive signal every second to keep UDP tunnels from collapsing

Some routes might drop UDP tunnels through NAT firewalls after a very
short time frame. This leads to data being sent to the corresponding port
being lost. To prevent this every second a small mtu probe packet is sent
in order to keep the tunnel open and working. This does not eliminate the
possibility of packet loss due to eager routers clearing their tables, but
reduces the packet loss significantly.