FetchRemote - porting from ruby to go under feature flag

As we are on the roads of removing ruby code from our codebase it is a next step in porting of ruby implementations into go code. FetchRemote RPC now has go implementation that could be enabled by setting a feature flag 'gitaly_go_fetch_remote'. To track usage of the new implementation a new counter 'gitaly_fetch_remote_counter' was added (temporary until both implementations exist). The change also includes more extended testing.

Closes: #1466 (closed)