Refactor HistoryTree into NonEmptyHistoryTree and HistoryTree

[conradoplg]

Motivation

While working on #2135 I had to use Option<HistoryTree> in a lot of places since the history tree is only created in the Heartwood activation block. I also had to add the same logic in multiple places to create the tree when that block is reached.

Specifications

N/A

Designs

N/A

Solution

This renames HistoryTree into NonEmptyHistoryTree and adds a HistoryTree which is basically a Option<HistoryTree>, and adds a method to push a block and create the tree when needed.

Review

This will block the upcoming #2135 PR.

@teor2345 and @dconnolly will be more familiar with the code being refactored.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

Follow Up Work

Will be used by the upcoming #2135 PR.

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This change is 

[teor2345]

I have some questions, and some suggestions for follow-up refactors.

But I can't see anything blocking.

[teor2345]

All my questions got answered, fixed, or have open tickets for follow up.