Use the new Gitaly auth scheme (v2)

Part of https://gitlab.com/gitlab-org/gitaly/issues/1319

This MR makes workhorse use the new authentication scheme discussed in https://gitlab.com/gitlab-org/gitaly/issues/1305 based on an HMAC token. The current production gitaly version takes both this new method and the old one as valid, so there should be no compatibility issue. The old authentication scheme will be deprecated only once all clients are using the new auth scheme.