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

Hooks were run for wikis, but given the internal API wasn't responding
failures happended, as seen on:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18693

When `Errno::EBUSY` is raised during restore, this could indicate that
the directory being restored into is a mountpoint. In this case we
explain the user how to retry the restore.

Instead of renaming existing storage directories to a timestamped
format, create a timestamped directory in a temporary location and move
the individual files and folders inside storage directories to it. This
helps us with nested storage directories.