SQS alarms

[gclitheroe]

There are SQS queues for archive upload notification messaging. This includes dead letter queues. We need to decide what (if anything) to alarm on and where to send the notification in alarm state.

Typical alarm states that are worth notifying for are:

• any messages on the dead letter queue (dlq). These could not be processed for some reason and after a small number of delivery attempts are moved to the dlq. I have had issues with alarms on dlq sitting in an "insufficient data state" because nothing is happening.
• messages backing up on a main messaging queue which can mean the consumer has stopped or is not keeping up for some reason.

@quiffman @ozym @nbalfour - it would be good to have some discussion about what to do then we can get this ticket to a ready state.

[gclitheroe]

This will no longer be relevant when ram cache pkgs etc are used.