Checksum calculation for a repository

Computing the repository checksum (https://gitlab.com/gitlab-org/gitlab-ee/issues/4755, https://gitlab.com/gitlab-org/gitlab-ee/issues/4756) will be relatively expensive and should be ported to Gitaly.

/cc @stanhu @nick.thomas

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information