Support restart of partial download

Downloads of very large images may be interrupted prior to completion, due to dropped connections and such.

Rather than discard the partial image and start again from the zero offset, it could be preferable to restart from the current offset where the storage backage support range-based GETs and then knit the remainder of the image content into the partial result of the first GET.

In order to avoid polluting the API, this logic should be limited to the service->backend leg (as opposed to end-to-end interaction from the client side).

The change should be restricted to the glance backend store gluecode for those
stores that support Ranges (S3, Swift, HTTP).

In the S3 case, the boto.s3.resumable_download_handler already provides restartable logic in a fairly generic way, similar logic could be applied to the Swift and HTTP backends.

UPDATE:

At the J summit, we discussed adding this to both, the http and the store, APIs.