Robustness fixes

Create queues durably, so that messages aren't lost between a message broker restart and the start of our queue consumer.

Disable advance fetching of messages, so that one worker doesn't starve others.

This is the remaining robustness work for https://www.pivotaltracker.com/n/projects/1110932/stories/80019104
This is a good call, though the channel method now has two responsibilities and its intent is "cloudy" enough to warrant  7 lines of comment.

How do you feel about adding a `NUMBER_OF_MESSAGES_TO_PREFETCH` constant and changing `#channel` to 
`@_channel ||= create_channel_with_set_message_prefetch_count` ?

This would be a private method that creates the channel, sets the prefetch and returns it.