Add node disconnecting from base logic

The nodes can now detect that the base has been disconnected and
they will starting searching for new base. They rely on a cmd
sent to server (NODE_KEEP_ALIVE) receiving a reply back under the
time limit (2000 loops). Every NODE_KEEP_ALIVE sent from server
contains the status of the relay pin, so use this to update the
node_status on the server side. This will cause less traffic than
an explicit NODE_STATUS command although that functionality is
still there.

Issue: #15