Make admin websocket more functional

[FZambia]

The goal here - make admin websocket endpoint (/socket) more advanced and similar to client endpoint in the end. This should not affect users as our web interface is embedded and will be updated together with Centrifugo.

• Rename auth to connect
• Support API commands (so it will be possible to remove http ActionsHandler)
• Call stats API instead of InfoHandler (so remove InfoHandler)
• Possibility to start communicating with admin endpoint without auth (--insecure_admin, maybe rename --insecure_web to --insecure_admin, but leave alias for backwards compatibility)

Benefits:

• publish over pool of Websocket connections
• less various HTTP handlers that were only required for web interface

The only problem I see at moment - how to show secret and current configuration options - so it seems extra admin method will be required to get current config.

[FZambia]

The work started here and first changes appeared in 1.4.2 release. Though still more work needed to make admin websocket able to be an way to work from application backends - need to add queue to admin connection similar to client's queue.

[FZambia]

Message queue added in 71faa51