Support default exchange in consumer and publisher modules

[mkorszun]

As requested in https://github.com/meltwater/gen_rmq/issues/206, this PR introduces support for the default exchange in the publisher and consumer module.

Default Exchange

The default exchange is a direct exchange with no name (empty string) pre-declared by the broker. It has one special property that makes it very useful for simple applications: every queue that is created is automatically bound to it with a routing key which is the same as the queue name.

Implementation:
Whenever a publisher is configured with the default exchange, we need to ensure that such an exchange is not redeclared otherwise broker will return an error.

Whenever a consumer is configured with the default exchange, we need ensure that exchange is not redeclared and binding is not created (it already exists).

The default exchange is configured by providing :default atom as an exchange:

def init() do
  [
    exchange: :default
  ]
end

Description

Checklist

• I have added unit tests to cover my changes.
• I have improved the code quality of this repo. (refactoring, or reduced number of static analyser issues)
• I have updated the documentation accordingly

[coveralls]

Coverage remained the same at 89.967% when pulling d62e8eb on support-default-exchanges into e67a6ee on master.

[coveralls]

Coverage remained the same at 89.369% when pulling 98a7757 on support-default-exchanges into 28f98d5 on master.

[vorce]

Looks really nice @mkorszun 👍

[vorce]

Can we add something that explains :default in the documentation here as well? I think it would be the first place I would look.

[mkorszun]

@vorce good point. Will update / extend the documentation.

[mkorszun]

@vorce added some docs. Let me know if this is smth you had in mind.

[vorce]

@mkorszun yes, perfect 👍

[akoutmos]

Looks good to me!