Introduce priorities for subscribers

[jollopre]

A subscriber, also known as consumer, receives messages from the queue is subscribed to. Since a harmoniser process can have many subscribers running simultaneously, it might be interested to priority the delivery of messages to the subscribers. More info about this feature can be found here.

The prioritisation of higher consumers only takes place if the number of acknowledged messages is lower that the prefetch configured. That means subscribers not only have to be configured with manual ack but also have to set a prefetch count.

Although this feature can work independently of the concurrency option available for harmoniser, we believe is more suited for bounded concurrency configured so that under a process that already shared a number of threads, some subscribers are more prioritised than others.