Replace pollQueue with new built-in SQS events

[lmorchard]

We built a pollQueue function invoked by a CloudWatch alarm every 60s to process SQS messages. But, Amazon has just added an SQS event source driven by an in-house queue poller that should be able to replace our hacky pollQueue.

The Amazon queue poller processes SQS messages as quickly and with as many parallel invocations of Lambda functions as limits allow. Need to consider how this could be configured to respect rate limiting, which is kind of our main goal here.

Some further relevant links / notes:

• New support in Serverless framework - included in the recent 1.28.0 release
• https://www.jeremydaly.com/serverless-consumers-with-lambda-and-sqs-triggers/

[lmorchard]

This one is actually done!

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