Desktops advertise client-keepalive-v1; the edge offers it after the hello-ack and
auto-responds to a fixed keepalive frame, so idle hosts need no 25 s ping alarm
(each setAlarm is a billed row write). Silent keepalive hosts are probed with one
relay ping before a phone is refused, closed at once where the host is leaving.
Shipped desktops and cells see nothing new. Kill switch CLIENT_KEEPALIVE=0.
Neil
cc7d19e932