[storage] Block Pruning Optimization

[patrick-ogrady]

This PR improves significantly improves block pruning performance by flattening the storage of transaction records. This new modeling approach means we no longer need to fetch a record before we prune it!

Changes

• flatten transaction data model
• concurrent transaction pruning
• reduce number of fetches needed to prune
• pass existing tests
• cleanup variable naming

[coveralls]

Pull Request Test Coverage Report for Build 13016

• 73 of 82 (89.02%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 78.411%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
storage/modules/block_storage.go	73	82	89.02%

Files with Coverage Reduction	New Missed Lines	%
storage/modules/block_storage.go	3	78.03%

Totals
Change from base Build 12970:	0.3%
Covered Lines:	7914
Relevant Lines:	10093

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💛 - Coveralls