Update routes to reflect specification changes

To bring the implementation inline with the specification, the names and
structure of the API routes have been updated.

The overloaded term "image" has been replaced with the term "manifest", which
may also be known as "image manifest".  The desire for the layer storage to be
more of a general blob storage is reflected in moving from "layer" api prefixes
to "blob". The "tarsum" path parameter has been replaced by a more general
"digest" parameter and is no longer required to start uploads. Another set of
changes will come along to support this change at the storage service layer.