Refactor ad-hoc protobuf marshaling in gitaly test server

Our tests check if workhorse sends correct gitaly requests to gitaly by starting a mock gitaly test server. This mock server would then marshal the gitaly request, clumsily, and inject it into the http response.

This change switches to using standard jsonpb protobuf JSON marshaling instead of ad-hoc marshaling. It skips the mock PostReceive handler because that one is already refactored in !281 (merged).