Comm: reset base_mode, custom_mode & status on connection lost

If the link with the UAS is lost, receivedMode is turned to false. Yet when the link is regained and the HEARTBEAT message is received (with the same base_mode) receivedMode will stay false and prevent UAS::setMode() from sending mode update to the UAS.

I am not sure this PR is the correct or best one, but it does address the issue.