One Gitaly connection per storage shard

What does this MR do?

Part of https://gitlab.com/gitlab-org/gitaly/issues/144

This paves the way for running Gitaly across the network.

Because storage shards may eventually point to different Gitaly servers, we must change the Gitaly address from a global configuration attribute to a per-storage attribute.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We need to be able to route Gitaly requests to the correct Gitaly servers.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitaly/issues/144