This change adds abstractions for opening and managing TCP connections
primarily for use with the STOMP broker support. As one immediate
benefit the change makes the  StompBrokerRelayMessageHandler more
easy to test.

Issue: SPR-10835

Consolidates new, messaging-related classes from spring-context and
spring-websocket into one module.

org.springframework.websocket  -> org.springframework.web.socket
org.springframework.sockjs     -> org.springframework.web.socket.sockjs

Flatten .sockjs and .sockjs.server

Add HandlerProvider<T> class
Modify HandshakeHandler to accept + adapt WebSocketHandler at runtime
Modify SockJsService to accept + adapt SockJsHandler at runtime