Evaluate whether sequencer is required now SQS does FIFO

[kenoir]

The purpose of sequencer was to provide a FIFO interface to SQS, this is now available directly from AWS (http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/FIFO-queues.html).

It might be useful to evaluate the usefulness of this gem, and refactor to remove the underlying complexity if it is not required.

[jamesdonoh]

This is a very appealing idea, however even with FIFO queues I wonder whether all notification providers (such as CPS) can contract to push notifications onto the queue in strictly sequential order. If not then we may still need logic in the renderer to handle "non-sequential" messages.