NonFinalizedState: Finish implementation and testing according to RFC0005

[yaahc]

Motivation

This PR is tracking the follow up changes necessary to fully implement the
non-finalized state according to the state RFC and to test the implementation.

• Correctly handle duplicate blocks in CommitBlock
• test the best chain is selected correctly
• test the best chain height is correctly returned
• check that the right queued blocks are pruned

Related Issues

• Prior PR - Implement MVP of NonFinalizedState and integrate it with the state service #1101
• Relevant comments - Implement MVP of NonFinalizedState and integrate it with the state service #1101 (comment) and Implement MVP of NonFinalizedState and integrate it with the state service #1101 (comment)
• PR updating RFC - State RFC: Handle duplicate block edge cases #1136

[teor2345]

This refactor is good as it is, and I would like to merge it straight away.

(If we keep building on it, then any future reviews will have to read through this refactor code - even though we know it's good.)

I think the TODO list in this PR should actually become a tracking issue.

[teor2345]

I think the TODO list in this PR should actually become a tracking issue.

Or maybe a section in the RFC5 tracking issue - whatever works best for the developers and reviewers.

[teor2345]

This is good work, and it's an improvement, so I'm going to merge it now.

I moved the remaining TODOs into the state tracking issue #1049.