Deprecate ListDirectories RPC

This RPC is only used in cleanup rake tasks, as far as I can tell. E.g. https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/tasks/gitlab/cleanup.rake#L7

This is problematic for Gitaly HA because we there will be directories on multiple different Gitaly servers and we don't want to directly expose those Gitaly servers.

We should take a look at the things that use ListDirectories, and reimplement whatever it is they do inside Gitaly. In some cases it might be automated cleanup; in other cases we might need a new RPC. In the latter case that RPC should express what we want to happen, and leave the actual work to Gitaly.