Add method for sending msgs to SQS that look like they're from SNS

* Adds method "postBatchAsSNSMessages" to QueueService, that sends batches of messages to the destination queue, where all of the messages look like they were sent from SNS.
* In effect, the method will add the SNS envelope around the sent messages.
* Refactor "postBatch" to use shared methods from "postBatchAsSNSMessages".