Renamed Gitaly services

What does this MR do?

Closes https://gitlab.com/gitlab-org/gitaly/issues/379

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

GitalyClient classes:

• Gitlab::GitalyClient::Commit -> Gitlab::GitalyClient::CommitService
• Gitlab::GitalyClient::Notifications -> Gitlab::GitalyClient::NotificationService
• Gitlab::GitalyClient::Blob -> Gitlab::GitalyClient::BlobService
• Gitlab::GitalyClient::Ref -> Gitlab::GitalyClient::RefService

Gitaly protobuf generated classes:

• Gitaly::Diff -> Gitaly::DiffService
• Gitaly::Ref -> Gitaly::RefService
• Gitaly::Commit -> Gitaly::CommitService

Why was this MR needed?

To match a similar rename in Gitaly.

Screenshots (if relevant)

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?