public synchronized void registerEchoCallback( @Nonnull final Packet stanza, @Nonnull final CompletableFuture<?> result )
        {
            if ( stanza.getElement().element(FMUC) == null ) {
                throw new IllegalArgumentException( "Argument 'stanza' must have an FMUC child element." );
            }

            Log.trace( "Registering callback to be invoked when peer '{}' echos back stanza {}", getPeer(), stanza.toXML() );
            pendingEcho.add( new PendingCallback( stanza, result ) );
        }
A list of stanzas that need to have been echo'd by a remote FMUC node, before they can be processed locally.
This collection _does not_ include stanzas needed to set up the initial join. This collection is only used
for subsequent stanzas that are shared in a setting where echo-ing is required (due to the mode of the
federation being defined as 'master-slave').
xmppserver/src/main/java/org/jivesoftware/openfire/muc/spi/FMUCHandler.java