Add Tarsum Calculation during v2 Pull operation

While the v2 pull operation is writing the body of the layer blob to disk
it now computes the tarsum checksum of the archive before extracting it to
the backend storage driver. If the checksum does not match that from the
image manifest an error is raised.

Also adds more debug logging to the pull operation and fixes existing test
cases which were failing. Adds a reverse lookup constructor to the tarsum
package so that you can get a tarsum object using a checksum label.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)