Support priority queue declarations

[ebardes]

I'm working on project that benefits from using priority queues in RabbitMQ (available since version 3.5.x). I feel that others might benefit from this functionality too.

[michaelklishin]

@ebardes you already can use priority queues with any client that supports optional queue arguments. While I couldn't quickly find an example of how to do that with this client in either examples or tests, I'd be really surprised if amqp.node didn't provide a way to do that.

[ebardes]

I couldn't find a way to declare a queue with the 'x-max-priority' flag. Perhaps I just missed it or that I'm not familiar enough with the nuances of amqp.node yet.

If you declare a queue through the management interface, the runtime code errors because the queue declarations don't match.

[michaelklishin]

Ah, so there's known argument filtering going on.

[ebardes]

Exactly. I just added another known argument.

[nikosd23]

This is very important for us! When do you plan to perform a new release with this included?

[michaelklishin]

@squaremo this PR looks good to me.

[nikosd23]

Tested with RabbitMQ 3.3.4 broker and it seems to create the queues successfully, ignoring the x-max-priority option making the change backwards compatible

[squaremo]

Ah, so there's known argument filtering going on.

No there's not .. I see why you'd think that though. The second argument to #assertQueue is options, and an x-* argument would go in the arguments table in the options:

ch.assertQueue('prio', {arguments: {'x-max-priority': 10}});

whereas the "known options" go in the top level:

ch.assertQueue('prio', {maxPriority: 10});

Anyway, yes this PR looks fine.

[michaelklishin]

@nikosd23 all unknown arguments will be ignored. They are all optional.