feat(kv): define forward cursor interface

[GeorgeMac]

Closes #5367

This PR introduces the concept for a ForwardCursor as described in the issue above. It's purpose is to create a contract in cursor creators declare upfront the direction and start position in which to create a cursor. This will promote healthier access patterns to the existing cursor. Allowing us to optimize a lot easier in the kv bucket implementations.

1. Define the forward cursor interface. A single direction cursor which is configured with a direction on creation.

c := bucket.ForwardCursor([]byte("runs/"),
        kv.WithCursorDirection(kv.CursorDescending),
        kv.WithCursorHints(kv.WithCursorHintKeyStart("runs/12345")))

2. Update all bucket implementations to adhere to new ForwardCursor() method.

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass
• http/swagger.yml updated (if modified Go structs or API)
• Documentation updated or issue created (provide link to issue/pr)
• Signed CLA (if not already signed)

[stuartcarnie]

Looks great George. I found that that inmem goroutine for the ForwardCursor can leak if the consumer does not call Next to completion.

[stuartcarnie]

Great improvements – one minor suggestion to simplify the Close logic

[stuartcarnie]

Awesome stuff – this is the beginning to improving the K/V APIs 🏆