Add support for batching reports to the Pyzor server.

For long running daemons/operations (such as the new forwarder system), the Pyzor client could be improved to batch the reports (or whitelist) to the server. The server will also need to adjust it's protocol to accept multiple digests.

ISTM that the simplest way to do this is by appending more headers `Op-Digest` headers to the request.
