Report newly transformed files

[BenGalewsky]

Problem

The client can only ascertain newly uploaded transform results by polling S3 bucket and see what objects have appeared since the last poll. This puts a strain on the Object Store server and doesn't scale once the bucket gets large.

Approach

The transform status REST endpoint can optionally report the files that are generated as part of the transform, but it needs the following enhancements to be useful in this context:

• Need to record the hashed/shortened filename so that it's easy for the client to download from the object store
• Need to record a timestamp of when the result was uploaded so the REST endpoint can support a ?filesSince parameter. The client will poll for new files since the last successful request
• The transformer needs an essential fix to only report the file once the upload to S3 is complete