XD-447 Add MQTT Sink

Parameters:

`clientId` (default 'xd.mqtt.client.id.snk' from mqtt.properties (+.snk))
`topic` (default 'xd.mqtt.test' from mqtt.properties)

See mqtt.properties for other properties such as broker
url.

To test:

Install RabbitMQ mqtt plugin

Create two streams:

'time --interval=5 | mqtt'
'mqtt | log'

Note: mqtt requires unique client ids; the mqtt source has been changed so
that its default clientId is now 'xd.mqtt.client.id.src' from mqtt.properties (+.src)).