Multichannel ZMQ Collection from Cento/n2disk

[cardigliano]

When processing high speed links with cento and n2disk, ntopng receives many flows/sec on the ZMQ channel, leading to flow drops when this exceeds a certain rate, especially when they are sent in bursts. It is required to use multiple ZMQ channels to mitigate this.
In addition to this it is requested to mitigate export bursts in n2disk using an export queue (see ntop/n2disk#32)

[cardigliano]

In cento it is possible to load-balance flow export via zmq using a comma-separated lists of endpoints, this can also be used to setup multiple channels towards the same endpoint (configured as collector)