Add historical balance lookup

[patrick-ogrady]

Motivation

• Calls to fetch an account balance in the fetcher did not allow for providing a PartialBlockIdentifier for a historical balance query
• Calls to fetch an Account balance in the fetcher did not return account metadata

Solution

Fixed both issues. Also added an assertion to ensure that historical balance query responses contain a BlockIdentifier related to the query PartialBlockIdentifier.

Miscellaneous

Once this is merged, it will be possible to perform rosetta-validator reconciliation while syncing.

[coveralls]

Pull Request Test Coverage Report for Build 569

• 22 of 22 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 89.855%

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Totals
Change from base Build 562:	0.4%
Covered Lines:	434
Relevant Lines:	483

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💛 - Coveralls