Use one RabbitMQ connection + use co-routines

[angrave]

TODO: TaskEngine should use one RabbitMQ connection for the whole process not one per task
TODO: Take a deep dive into how this actually working and document it.

It would also be useful to confirm that we are actually using async co-routines and not multiple threads.

TaskEngine does not explicilty create any threads; however it certainly has a main loop that sleeps for a couple of hours, while mulitple message queues are being concurrently services!

We may need to upgrade to the latest RabbitMQ C# client.

And emperically confirm that RabbitMQ C# Implementation with a prefetch count>1 does not implement concurrency using multiple threads.
e.g. alway print out a threadID and print out how many threads are running
e.g. take a deep dive into the C# RabbitMQ source code.

WebAPI/CTCommons/RabbitMQ/RabbitMQConnection.cs

Line 129 in e53313c

consumer.Received += async (model, ea) =>

_logger.LogInformation("Prefetch concurrency count {0}" , concurrency);

            _channel.QueueDeclare(.... );

            _channel.BasicQos(prefetchSize: 0, prefetchCount: concurrency, global: false);
        }
        var consumer = new EventingBasicConsumer(_channel);
        consumer.Received += async (model, ea) =>
        { ...