To shutdown all UIs, one has to "stop" the heart of the UI session.
The heart of a UI session is a database relation in the
"session.db" database. It is emptied when a shutdown message is
received by a running WebSocketProtocol instance.

All other connections periodically check the heart and update its
timestamp. When the tupple containing the session id is empty, all
connections of the same sesion die.

The WSGI server, which is running as a member of a SessionHelper
instance, is shut down by a simpling threaded method launched by
the SessionHelper before running the server. This method is called
_shutdown_daemon and is polling the database every 4 seconds. When
the database is empty, it sends a shutdown signal to the WSGI
server.

These changes affect the way an Agkyra daemon is launched, as well
as the "launch_daemon" and "stop_daemon" CLI commands.