Allow the mesh to detect when a node has completely restarted.

When calling meshlink_open(), a node creates a unique ID that is passed
along ADD_EDGE messages. When a node becomes unreachable and then reachable
again, this allows other nodes to detect whether it was just a temporary
network issue, or whether the node completely restarted (either because
meshlink_close() was called or because it crashed).

At the moment, when this is detected, other nodes close all open channels
with the restarted node.