Merge current state of Notification Stream from dev/ntf-stream branch

There is a new dedicated socket for change notifications that
the Server listens on. This socket is called Notification Bus
and introduces a new way of handling change notifications on
the Server. There is now only a single thread that handles all
notification subscribers (instead of current one thread per
subscriber), which should reduce memory footprint of the Server
without impacting the performance because we don't need to handle
multiple incomming request at once.

Every Monitor creates a new connection to the Notification Bus
and it now uses the Bus to manage its notification subscription
settings using newly introduced Protocol commands. The Server
uses the same connection to send change notifications back to
the Monitor. Since we are using the Protocol to communicate over
a dedicated socket, we now have a dedicated Protocol command for
each notification type (Item, Collection, ...).

All this lays the foundation for further work on Notification
Payloads (sending the relevant Item/Collection/... as part of
the Notification) and Server-side Change Recording.