API support for random access to images

This patch adds support for random access to images to the API v2. It's
possible to partially access the image data by passing the offset where
the image should be fetched from and the size of the chunk expected.

Note that not all stores support random access to image data. In such
cases, the API will return an error mentioning the selected store
doesn't support such a feature.

DocImpact:
Content-Range header is now parsed to support random access to image
data.

Implements blueprint: restartable-image-download

Change-Id: Iade692a8197a9a1d9532711a5c572c505e5e7d20
Merge "API support for random access to images"