relax coin amount in fast-forward logic

[arvidn]

Update the fast-forward feature to allow the coin amount to change when "rebasing" a spend onto a later version of the same singleton.

[coveralls-official]

Pull Request Test Coverage Report for Build 7226536921

• 20 of 20 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 86.061%

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Totals
Change from base Build 7222544523:	-0.02%
Covered Lines:	10854
Relevant Lines:	12612

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💛 - Coveralls

[AmineKhaldi]

I see two remaining tasks with this patch:

1. We need to account for (and cover with tests) the fact that not only the latest version's amount can change, but its parent's amount can change too.
2. We need to relax (and cover with tests) the check for output coins as it still requires the same amount.

[Rigidity]

lgtm