backlog property is setup to be read from the configuration and used in the Acceptor, but is not in the allowableAcceptorKeys which leads to a warning logged and JBoss saying it is ignoring it.

artemis-core-client/src/main/java/org/apache/activemq/artemis/core/remoting/impl/netty/TransportConstants.java: public static final String BACKLOG_PROP_NAME = "backlog";
artemis-server/src/main/java/org/apache/activemq/artemis/core/remoting/impl/netty/NettyAcceptor.java: backlog = ConfigurationHelper.getIntProperty(TransportConstants.BACKLOG_PROP_NAME, -1, configuration);
artemis-server/src/main/java/org/apache/activemq/artemis/core/remoting/impl/netty/NettyAcceptor.java: bootstrap.option(ChannelOption.SO_BACKLOG, backlog);