[WIP] Add option for making consumer exclusive

[anderslemke]

• Documentation

In some situations, the execution order of messages is crucial.
In such situations, RabbitMQ can ensure that there are no more than one consumers on a queue, with the exclusive option.

[michaelklishin]

This does not make the consumer exclusive, it makes the queue exclusive. Is that what you are really after?

[anderslemke]

@michaelklishin I might confuse some concepts here.

There is an exclusive flag in this gem already:

sneakers/lib/sneakers/configuration.rb

Line 20 in 7c059d1

:exclusive => false,

As far as I understand, this controls the queue: https://www.rabbitmq.com/queues.html#exclusive-queues . This is what I would call "exclusive queue".

I want this PR to control the exclusive-flag in https://www.rabbitmq.com/amqp-0-9-1-reference.html#basic.consume . This is what I refer to when I say "exclusive consumer".

I will beef up the PR-description to talk about the motivation for this.

[anderslemke]

@michaelklishin I've tested this PR locally, and I get the following results.

When I set the new exclusive-flag to false, I get consumers with this config:

When I set the new exclusive-flag to true, I get a consumer with this config:

This is actually what I want to achieve. But I'm not sure I use the terms correct, though.