service: add watchdog timestamp

service: add watchdog timestamp

This patch adds WatchdogTimestamp[Monotonic] to the systemd service
D-Bus API. The timestamp is updated to the current time when the
service calls 'sd_nofity("WATCHDOG=1\n")'.
Using a timestamp instead of an 'alive' flag has two advantages:
1. No timeout is needed to define when a service is no longer alive.
   This simplifies both configuration (no timeout value) and
   implementation (no timeout event).
2. It is more robust. A 'dead' service might not be detected should
    systemd 'forget' to reset an 'alive' flag. It is much less likely
    to get a valid new timestamp if a service died.