[WiP] Process with Batch

[mojodna]

This refactors the image processing worker to optionally use AWS Batch to delegate processing to a Spot Fleet (managed by Batch).

Pending:

• [ ] try it out
• [ ] (oam-uploader-api) implement a web hook to allow images to be marked as done remotely
• [ ] (oam-dynamic-tiler) call web hook when processing is complete
• [ ] consider race condition where metadata will be available in the S3 bucket prior to associated images
• [ ] cloud-free parity using oam-dynamic-tiler's Docker image (or local version of process.sh, if this is running inside a Docker container; requires that appropriate version of GDAL is available)
• [ ] misc. cleanup
• [ ] document environment variables for Batch

[mojodna]

Oh, forgot to mention that this generates UUIDs for imagery and drops upload dates from the S3 path.