Use gitaly-upload-pack and gitaly-receive-pack

Closes https://gitlab.com/gitlab-org/gitaly/issues/193

We want to use Go Gitaly client executables in gitlab-shell because we cannot reliably use Ruby gems with C native extensions in Ruby executables launched by SSHD. This change adds such Go executables, for now without Gitaly support but that should change soon.

This change makes bin/compile a mandatory installation step. All git pull and git push sessions over SSH will start flowing through these Go executables.