Replace `::StorageShard` with `::Shard`

Per https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23482#note_123362550

EE has a virtual model called StorageShard, but CE has since introduced a "real" Shard model and associated table. We should remove the EE StorageShard and migrate its unique functionality (just calculating a digest of sorted shard names) to the new model, and possibly to CE.

Since the main user is Geo at present, which looks for differences between what is in the database and what is on disk, we may need to introduce the idea of which in-database shards are currently specified on-disk as well.