[chore] Miscellaneous Nits Before Release

[patrick-ogrady]

This PR fixes a few miscellaneous issues found while integrating rosetta-sdk-go into rosetta-cli (coinbase/mesh-cli#193).

Changes

• Add CounterStorage.GetTransactional
• Fix negative big.Int regression: https://play.golang.org/p/wD68BU8AACd
  • find issue in docs: https://golang.org/pkg/math/big/#Int.Bytes
  • reproduce with test
  • don't store pruned if < 0 (otherwise will appear as positive when reading)
• remove erroneous unlock
• delete all historical balances in deleteAccountRecords

[coveralls]

Pull Request Test Coverage Report for Build 12936

• 13 of 19 (68.42%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 78.147%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
storage/modules/balance_storage.go	13	15	86.67%
storage/modules/counter_storage.go	0	4	0.0%

Totals
Change from base Build 12840:	-0.03%
Covered Lines:	7896
Relevant Lines:	10104

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💛 - Coveralls