asserting dead-letter exchange and queue

[ujwal-setlur]

This is a PR to assert the dead-letter exchange and queue in the amp adapter, to address https://github.com/moleculerjs/moleculer-channels/issues/67

The PR asserts the exchange and queue during subscription of the first channel. This was tested by throwing an exception in the channel handler and checking the dead-letter queue on RabbitMQ.

[ujwal-setlur]

@AndreMaz Thanks for the comments. I will review.

[ujwal-setlur]

@AndreMaz I have addressed comments. I will add a test as well.