FC-40 Clean up SQS message handling

This is preparatory work for adding start/shutdown control later.
Removed RejectedExecutionHandler by controlling when messages are received from the queue. A semaphore is used to manage the number of messages being handled by the thread pool. Also corrected behaviour to receive up to 10 messages per SQS request, rather than 1.

Signed-off-by: Steffan Westcott <steffan.westcott@clicktravel.com>