Changes

Northbound websocket improvements

This commit makes several improvements to the northbound websocket interface:

* Authentication using the sensinact Authenticator services
* Improved ordering guarantees. Websocket clients will always receive a subscription response before receiving their first notification for that subscription
* Thread safety - the web socket session was not updated in a thread safe way so notifications would sometimes fail with NPE

Signed-off-by: Tim Ward <timothyjward@apache.org>
 (commit: 3001cde)