Switch from returning Mono<WebSocketSession> to take a WebSocketHandler
and return Mono<Void> for the entire session handling.
The WebSocketHandler callback delimits the stard and end of protocol
handling and forces the handler to operate within the scope of the
Reactor operators.
Give the full duplex nature of WebSockets, the symmetry between client
and server (each now using WebSocketHandler) also seems appropriate.
Issue: SPR-14527
14068d5274