Implement notification subscription management via Protocol

NotificationManager now lives in separete thread. It creates
NotificationSubscriber object for each new connection on NotificationServer.
Each connection does not have its own thread as in case of Connection,
because the communication there is minimal and we don't mind if the
NotificationManager thread gets blocked for a short moment.

Each Subscriber keeps state of the subscription. When a notification is
delivered to NotificationManager, it will pass them to all subscribers
which will then decide whether they accept the notification and eventually
send it to subscribed client.

Currently this breaks notification and subscriber debugging because we
completely removed the DBus introspection. The plan is to introduce a
subscription change notification (a type of Protocol::ChangeNotification)
that clients like AkonadiConsole could subscribe to to get information about
new or removed subscribers and their subscription state.