[Beacon] Reliable timestamps

Currently the behavior of beacons makes it difficult to reliably log exact time events occurred.

The server cannot use time when the beacon has been delivered, because the UA may delay delivery for unknown amount of time.

The page can't just embed event timestamp obtained on the client side in the beacon payload, because clocks on clients may be unreliable.


It would be very useful if the beacon request included age of the beacon, in seconds (perhaps as an HTTP header). This would allow the server to easily calculate `current time - age` as the time of the event.

-- 
regards, Kornel