Moved to OPT_OUT in 7d14b725, Now, by
removing the feature gates, this is an mandatory feature.

Related issues:
- https://gitlab.com/gitlab-org/gitaly/issues/526
- https://gitlab.com/gitlab-org/gitaly/issues/1194

Closes https://gitlab.com/gitlab-org/gitaly/issues/749
Closes https://gitlab.com/gitlab-org/gitaly/issues/1212
Closes https://gitlab.com/gitlab-org/gitaly/issues/1195