[core] Add SmackConfiguration.TRUELY_ASYNC_SENDS

This option is meant a quick and dirty hack until
XMPPConnection.sendStanza() throws a dedicated Exception in case the
connection's outgoing queue is full.

smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2009 Jive Software, 2018-2020 Florian Schmaus.
+ * Copyright 2009 Jive Software, 2018-2022 Florian Schmaus.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -2000,11 +2000,18 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
         }, timeout, TimeUnit.MILLISECONDS);
 
         addAsyncStanzaListener(stanzaListener, replyFilter);
-        try {
-            sendStanza(stanza);
-        }
-        catch (NotConnectedException | InterruptedException exception) {
-            future.setException(exception);
+        Runnable sendOperation = () -> {
+            try {
+                sendStanza(stanza);
+            }
+            catch (NotConnectedException | InterruptedException exception) {
+                future.setException(exception);
+            }
+        };
+        if (SmackConfiguration.TRUELY_ASYNC_SENDS) {
+            Async.go(sendOperation);
+        } else {
+            sendOperation.run();
         }
 
         return future;

smack-core/src/main/java/org/jivesoftware/smack/SmackConfiguration.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2003-2007 Jive Software, 2018-2020 Florian Schmaus.
+ * Copyright 2003-2007 Jive Software, 2018-2022 Florian Schmaus.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -387,4 +387,14 @@ public final class SmackConfiguration {
             }
         }
     }
+
+    /**
+     * If enabled, causes {@link AbstractXMPPConnection} to create a thread for every asynchronous send operation. This
+     * is meant to work-around a shortcoming of Smack 4.4, where certain send operations are not asynchronous even if
+     * they should be. This is an expert setting, do not toggle if you do not understand the consequences or have been
+     * told to do so. Note that it is expected that this will not be needed in future Smack versions.
+     *
+     * @since 4.4.6
+     */
+    public static boolean TRUELY_ASYNC_SENDS = false;
 }