Unify download stream generator

A single, unified generator for download streams should be possible, with roughly the following input arguments:

• a generator returning pairs (upload_id, path)
• files params (compress flag and pattern for filtering by filename)
• include_subdirs (recursive include subdir content)
• single_raw_file (if we should just stream one file, without encapsulating it in a zip bundle) The generator also needs to handle caching and orderly closing.