Sync to AWS

[djmitche]

Similar to the GCP sync implemented in #3185, we should be able to sync replicas to AWS's object storage.

The tricky bit here is that, unlike GCS and Azure Blob, S3 does not provide a compare-and-swap operation.

Some reading suggests that the easiest way to accomplish this would be to use DynamoDB as a lock over the "latest" object in the S3 bucket.