DAIS11 — A&E Keep alive supervision

When an A&E subscription has a non-zero keep-alive time, the server will
insure that the client receives a callback on the subscription minimally at
rate indicated by the keep-alive time, even when there are no new events to
report. By providing callbacks at a minimum known frequency, the client can
be assured of the health of the server and subscription without resorting
to "pinging the server" (the common practice of making periodic calls to a
server method, e.g. IOPCEventServer::GetStatus() ).

Add a new member keep_alive_time in the AlarmsAndEvents::Subscription::State struct