Fix spurious channel closures after meshlink_stop()+meshlink_start().

Fix spurious channel closures after meshlink_stop()+meshlink_start().

When restarting the mesh without fully closing the handle, channels should
continue to work MeshLink was only stopped briefly. However, we were
accidentily always setting the connection timeout when a node's UDP status
changed, which would cause channels that didn't have any unsent data to
time out.