Add a new Tunnel Health Checker. The health checker is configured to run about every 5th ICMP probe and checks the packet exchanger's counters in both directions. If *no* packets have passed in one direction, this will tear down the tunnel, leaving it to be restarted on the next epoll cycle. gfe-relnote: n/a (QBONE-only change) PiperOrigin-RevId: 275474401 Change-Id: I934c45ff35d855593518ce76f4f2d9326e269adb