The new call combines GetObjectInfo and GetObject, and returns an
object with a ReadCloser interface.
Also adds a number of end-to-end encryption tests at the handler
level.
startOffset was re-assigned to '0' so it would end up
copying wrong content ignoring the requested startOffset.
This also fixes the corruption issue we observed while
using docker registry.
Fixes https://github.com/docker/distribution/issues/2205
Also fixes#3842 - incorrect routing.
