systemd watchdog

One way to mitigate issues like #514 could be to use systemd's `WatchdogSec=` property to automatically restart an idle cloudflared. For this to be possible, cloudflared would have to send a `WATCHDOG=1` message to systemd when it has successfully processed a request. For better performance, cloudflared could send the ping only when necessary instead of after every request, using the `WATCHDOG_USEC` environment variable to determine when a ping is expected.