Refine logic to release WebSocket ByteBuffer via Callback with Jetty 12

[rstoyanchev]

In Jetty 12, WebSocket handling methods that take ByteBuffer also take Callback and that needs to be called to release the buffer. On the WebFlux side we could use our DataBuffer abstraction to wrap and carry the Callback so it can be called when released. On the WebMvc side we don't have anything equivalent and will likely need to make a copy.