Add plugin (and plugin support) for background processing of non-JP2s

[jechols]

When dealing with a huge non-JP2 image, the server can become... sluggish, let's just say. It would be very handy to be able to put all requests for a given resource into some kind of queue and generate a tiled, multi-resolution JP2 before serving up those requests.

The external images plugin sort of does this, but it is definitely not production-ready, it does its processing in a very weird hook (the id-to-iiif call), and it just shells out to the JP2 compression command (though this might be the easiest approach, it's certainly not ideal).

[acdha]

Thinking about this from the perspective of S3, @eikeon and I have talked about doing this on-demand where hitting a non-JP2 image would trigger a Lambda function to convert it, which seems really nice for this kind of bursty one-time activity.