Update default value for broker URL

TCP/IP port 1883 is reserved with IANA for use with MQTT.
Fix return code handling of AsyncMqttClient#connect

The return code indicating success will be null if AsyncMqttClient is being used.
Update to Graylog2 0.90.0
Exclude Log4j dependency from graylog2-inputs
Make sure that MQTT client ID is 23 characters or less

Some older(?) MQTT broker don't like client IDs which are longer than 23 characters.
So we have to hash the Node ID with Murmur3 (32 bits) to get a reproducible and most
likely unique, yet shorter client ID.
Move topic subscription from MqttClient into AsyncMQTTClientListener

The async client doesn't block operations (duh!) so we have to react (BINGO!) to
the "connected" event in the AsyncMQTTClientListener.