[storage] Balance Exemption Overhaul

[patrick-ogrady]

Because many blockchains apply rewards after the parent block is processed (usually before transactions in the next block are applied), adjusting the computed balance to equal the live balance of the parent block before applying balance changes doesn't guarantee that a computed account balance won't go negative (if a transaction in the new block spends the new rewards). This PR modifies the balance exemption logic to compare the computed balance after applying operations but before being checked for a negative value.

Changes

• Compare computed balance with exemptions after new operations are applied
• Add a negative intermediate value test

[coveralls]

Pull Request Test Coverage Report for Build 9810

• 30 of 37 (81.08%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 78.358%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
storage/balance_storage.go	30	37	81.08%

Files with Coverage Reduction	New Missed Lines	%
storage/balance_storage.go	1	76.09%

Totals
Change from base Build 9783:	0.04%
Covered Lines:	7053
Relevant Lines:	9001

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💛 - Coveralls