Multiple WebSocket standard support

WebSocketListener can work with multiple implementation of the protocol simultaneously. The negotiation ensures that each client connect to the implementation it supports or fail if the version requested is not supported.

To add an implementation, a class derived from vtortola.WebSockets.WebSocketFactory must be created, and registered in the listener before start:

var server = new WebSocketListener(new IPEndPoint(IPAddress.Any, 8006));
var rfc6455 = new vtortola.WebSockets.Rfc6455.WebSocketFactoryRfc6455(server);
server.Standards.RegisterStandard(rfc6455);

Note that different standards must have a different version number. It is possible to register different [Message Extensions] (//github.com/vtortola/WebSocketListener/wiki/WebSocketListener-Extensions) in each standard implementation:

var server = new WebSocketListener(new IPEndPoint(IPAddress.Any, 8006));
var rfc6455 = new vtortola.WebSockets.Rfc6455.WebSocketFactoryRfc6455(server);
rfc6455.MessageExtensions.RegisterExtension(new WebSocketDeflateExtension());
server.Standards.RegisterStandard(rfc6455)

Note that [Connection extensions] (//github.com/vtortola/WebSocketListener/wiki/WebSocketListener-Extensions) applies to the listener, no to particular standards (since all share the same endpoint).