Non polling method of getting changes

snapd currently has to be polled (via GET /v2/changes) to tell if something has changed. This is important for a long running client (e.g. GNOME Software) to tell if another client (e.g. the command line snap tool) has changed something.

snapd used to have a websocket based system that was removed due to it not working.

Now snapd has GET /v2/logs?follow=true which returns application/json-seq perhaps we could make a GET /v2/changes?follow=true?