Send `X-Agent-Message` header with a blob retrieval invocation in manifest response

[alanshaw]

Alter manifest API to send X-Agent-Message header with a blob retrieval invocation per RFC#79.

#15 ensures that the mapping from piece to location nodes is availabe, and #16 will ensure that valid delegations for all pieces are available. This ticket involves collecting the relevant delegations and creating invocations for retrieval, and adding them as HTTP headers to the GET /sp/piece_manifest call.