Retrieve manifest_text from API server. If that fails, emit a warning
and fall back to reading directly from Keep.

This gives the API server an opportunity to provide additional
metadata, like hints about where the data blobs are stored.
When retrieving a blob, if local Keep servers don't have it and the
locator has +K@xyzzy, try GET http://keep.xyzzy.arvadosapi.com/hash.
Merge branch '2070-read-remote-arvados'

closes #2070