Implement server-side notification filtering

Clients can use the new DBus methods in NotificationSource interface to
configure the filter. The server will then emit each notification only
to clients that are actually interested in.

This decreases DBus traffic generated by the server a lot and reduces
CPU use, because clients don't have to do the filtering anymore.

Clients that don't subscribe with new subscribeV2() method will be
delivered all notifications and have to do the filtering themselves
(backward compatibility)

(Merged and squashed from serverside-monitor branch)

REVIEW: 111816
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