Optimize block/unblock

[vincentfretin]

See #80
Now that get_publisher is optimized and cost is O(1) , prefer using it instead of iterating over publishers in the room to send the blocked/unblocked event to the blocked user.
Well this may not be a big optimization, this is more for code readability and also to avoid having something like

        for room in &joined.room_ids {
            notify_user(&event, &whom, switchboard.publishers_occupying(&room));
        }

if a publisher is in several rooms, see #55 (comment) for context.

I tested it with the naf-adapter-janus example, the messages are still sent:

[vincentfretin]

I merged this in master in networked-aframe fork https://github.com/networked-aframe/janus-plugin-sfu