blobs: detect when the contents of a given URL change

[snarfed]

When we get a blob URL that we already have, we should still fetch it and see if the contents are the same, and if they're different, update our stored CID etc.

This bit @aliceisjustplaying recently with https://ap.mosphere.at/atproto . She changed her profile picture, which Bridgy Fed saw, but the URL was the same as before, so it didn't get updated.