Acknowledge stream header to client in CreateBundleFromRefList

CreateBundleFromRefList does not acknowledge the header message sent as the first message in the stream to the client. The client starts immediately streaming the main request body to the server after sending the request header. This makes retrying write requests more difficult as the client may not be able to retry with another Gitaly as it may no longer have the data it is streaming. As part of &9097, we should change CreateBundleFromRefList to acknowledge the header to the client and have the client stream the data only after it has received the acknowledgement.