Add max SQS concurrency settings to app.py Lambda to avoid Cognito or Dynamo throttling.

Currently the app.py Lambda will scale linearly with the number of messages in the queue. To avoid quota limits with Cognito token retrieval and posting of items to Dynamo (less likely) we should institute some sensible event source concurrency limits https://aws.amazon.com/blogs/compute/introducing-maximum-concurrency-of-aws-lambda-functions-when-using-amazon-sqs-as-an-event-source/.