Detach server from node

[FZambia]

In version 1.6.0 we introduced registration of custom server implementation using plugin-like system but as time goes I don't see any benefit in this design - this only makes code more complicated. In this pr I removed support for registering many servers and what is good now is that Node now does not know anything about server. I think this is a right step to isolate components from each other.

Also I removed Mediator interface - I still hope that one day in future Centrifugo can be used as library but it should probably be designed from scratch - for now Mediator is just extra unused code.

This pr introduces breaking changes in internal packages API but no breaking changes for Centrifugo server users - everything should work the same way.

Also some other minor changes also included - various code quality improvements.

[FZambia]

At moment after this refactoring it's possible to start Centrifugo inside Go app with only raw Websocket endpoint in this way:

package main

import (
	"net/http"

	"github.com/centrifugal/centrifugo/libcentrifugo/channel"
	"github.com/centrifugal/centrifugo/libcentrifugo/engine/enginememory"
	"github.com/centrifugal/centrifugo/libcentrifugo/node"
	"github.com/centrifugal/centrifugo/libcentrifugo/server"
)

func main() {
	nodeConfig := node.DefaultConfig
	nodeConfig.Secret = "secret"
	nodeConfig.Namespaces = []channel.Namespace{
		channel.Namespace{
			Name: "public",
			Options: channel.Options{
				Publish:   true,
				Presence:  true,
				JoinLeave: true,
			},
		},
	}
	n := node.New(nodeConfig)

	e, err := enginememory.New(n, &enginememory.Config{})
	if err != nil {
		panic(err)
	}

	if err := n.Run(e); err != nil {
		panic(err)
	}

	serverConfig := &server.Config{}
	s, err := server.New(n, serverConfig)
	if err != nil {
		panic(err)
	}

	opts := server.MuxOptions{
		Prefix:       "",  // can be sth like "/centrifugo" in theory
		HandlerFlags: server.HandlerRawWS,
	}
	mux := server.ServeMux(s, opts)

	http.Handle("/", mux)

	if err := http.ListenAndServe(":8000", nil); err != nil {
		panic(err)
	}
}

So theoretically this can be a starting point to use Centrifugo as library - but this is out of scope of this pull request.

[banks]

Wow this is a lot of code changes to review.

Overall I think this is a great idea - I never really saw the value in the mediator and agree that making Centrifugo into a lib is a bigger design problem than just a few hooks here and there.

I like removing a lot of code! I've not yet had a chance to look through this really closely. I'm happy if it's been well tested to accept, but it comes with a big pinch of salt that I've not really looked closely :)

If you'd really like a super thorough review @FZambia, I can try to find some more time later int he week to try and get my head around this more.

[FZambia]

@banks Paul - thanks for looking, it was important to hear that you agree with main idea. Then I suppose I'll merge this at some point - but any comment before that can help of course.