Websocket: Avoid modifying protocol behaviour using URL params

[FZambia]

At this moment client that want to use JSON protocol should connect to:

ws://host:port/connection/websocket

To indicate an intent to use Protobuf protocol URL should look like:

ws://host:port/connection/websocket?format=protobuf

Also, to give server a tip about binary type of used data used inside frames:

ws://host:port/connection/websocket?format=protobuf&encoding=binary

A bit awkward and hard to explain/document.

The proposal here is get rid of URL parameters. Use explicit server configuration since the pattern when someone want different types of protocol/encoding for same server should be really rare. And can be solved by introducing another HTTP handler using this proposal.

On client side we will now have an explicit option with protocol type where applicable (at this moment Javascript and Go clients).

Ex.:

const centrifuge = new Centrifuge('ws://localhost:8000/connection/websocket', {
    protocol: 'protobuf'
})

This information will be negotiated with server using WebSocket subprotocol. So no URL param pollution.

For Centrifuge the user-payload encoding format is not important at all. So I think the encoding tip can be safely moved to Centrifugo (since v3 I suppose) and removed from Transport here.

[FZambia]

Done in 98f6388, will be part of v0.18.0