The CallbackFuture implements the Axis2 org.apache.axis2.client.async.Callback API
and will get registered with the Axis2 engine to receive the asynchronous callback responses.
This object is also responsible for taking the java.util.concurrent.Executor given
to it by the JAX-WS client and using that as the thread on which to deliver the async response
the JAX-WS javax.xml.ws.AsynchHandler.