ket session associated with given socket channel and user. The returned futures will return <code>null</code> on
 <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/Future.html#get()" title="class or interface in java.util.concurrent" class="external-link"><code>Future.get()</code></a> if the message was successfully delivered and otherwise throw <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/ExecutionException.html" title="