Linear scan of a repository with lots (200+) of tags causes pull IO timeout

This was originally reported at https://github.com/GoogleCloudPlatform/docker-registry/issues/22

It seems when performing "docker pull foo/bar:sometag" the repository performs a linear scan of ALL tags in "foo/bar".  When backed by object storage systems like GCS, this can take a long time.  It has broken image distribution for us.
