#9007 - improve and document IBlobsBackend

This MR includes a refactor of the blobs filesystem backend in a way that:

• all interactions with twisted.web.server.Requests are made in the blobs resource itself, and are not passed to the backend.
• all methods now return deferreds.
• documentation is added on how to implement a new backend.

Other info:

• Closes: #9007 (closed)