Gitaly should not use GitLab Shell token to authenticate with internal API but have its own access

When GitLab Shell secret is changed, users won't be able to commit changes (via SSH, HTTPS or UI) to their repositories unless they restart Gitaly. This is a bit weird because from an architectural stand point, Shell should only affect SSH related actions. The fact that a change in Shell secret affected ability to commit even via HTTPS and UI generally points towards a hidden dependency of Gitaly on Shell.