allow configuring the prefetch count

[jfromaniello]

This is a major change, the current behavior is to ACK a message as soon as is received.

I am proposing instead to ACK the message once is processed. With the prefetch parameter unset, this almost the same than the current behavior.. rabbitmq will report that the messages are not acked yet but it will keep delivering messages as before.

But this allow us to configure the prefetch count which is very useful. Let's say we know our worker can't handle more than 5 operations at the time, with this change you can initialize the server as:

const server = new AMQPRPCServer(connection, { prefetchCount: 5 });

More info about prefetch here.

In addition to this I am using reject if the command fail, this is an ack the message but mark it as rejected and does not re-queues it. This is helpful when you are looking at rabbitmq metrics for example.

[anton-kotenko]

Looks reasonable.

The only thing I'd like to ask is:
please bump major version of library in package.json.
To avoid surprises for somebody