Streaming API for attachment data

@nolanlawson:
> It would be nice to have a more efficient method of replicating attachments to/from Couch. Currently we use multipart for uploads and GET /db/doc/att for downloading (see pouchdb/pouchdb#3964 (comment) for why). It'd be nice to be able to stream and restart attachment requests.
>
>Emerging browser spec for background uploads/downloads: https://github.com/WICG/background-fetch

/cc @daleharvey @janl

Start by reviewing CouchDB's existing multipart attachment upload and GET /db/doc/att download paths, then read the linked background-fetch proposal and the referenced PouchDB discussion. Done would require a defined streaming attachment API that supports restarting requests, but the issue does not identify implementation files or tests.

Written by the indexing model from the issue text.