o open web socket session
 associated with given socket channel. The returned futures will return <code>null</code> on <a href="https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/Future.html?is-external=true#get--" title="class or interface in java.util.concurrent"><code>Future.get()</code></a>
 if the message was successfully delivered and otherwise throw <a href="https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ExecutionException.html?is-external=true" title="class or interface i