[BuildStream] CAS server resource names & instance names

[Jim MacArthur <jim macarthur codethink co uk>]

At the moment in CASCache.py (client side), we're making resource names 
starting with the text 'blobs':

        resource_name = '/'.join(['blobs', digest.hash, 
str(digest.size_bytes)])

Which would give us names like

'blobs/0defa...435/32'

our CASServer (used by the remote artifact cache) handles these fine. 
However, the spec says:

// For downloading blobs, the client must use the
// [Read method][google.bytestream.ByteStream.Read] of the ByteStream 
API, with
// a `resource_name` of `"{instance_name}/blobs/{hash}/{size}"`, where
// `instance_name` is the instance name (see above), and `hash` and 
`size` are
// the [Digest][build.bazel.remote.execution.v2.Digest] of the blob.

So, with the default instance_name of a blank string, we should have a 
leading slash on the resource name. If we add this in the client, the 
server will reject it at the moment. It's a similar situation for 
writing blobs.

It looks like the CAS server in BuildGrid already handles both cases. Is 
there anything important I'm missing here, or can I go ahead and correct 
the resource names in BuildStream?