Annotate *Request messages to differentiate between mutations and accesors

When Gitaly HA will be implemented, the need will be there to know what method is a write operation and what is not. In this merge request a simple way is used to annotate the Request messages.

It would've been better to annotate the RPC methods. However, there's no API now to access that information right now from either Ruby and Go. This is scheduled, but there's no reason to wait for that to be released given the current approach works too.