amqp-consume: support consuming N messages at a time

If you have a single consumer C₁ and 10 messages are published, all 10
will be streamed to that one customer. Assume each message takes a few
minutes to be handled.

If a second consumer C₂ comes up before C₁ is able to process its first
message, it will stay idle until new messages are published, while C₁
will still have to process the other 9 messages after finishing with the
first one.

If both consumers were started with `--messages 1`, C₁ would only fetch
a single message, and start handling it; C₂ would start and already
receive the second message .