Adopt banker's rounding for interest calculations

[turadg]

What is the Problem Being Solved?

As yet we have two rounding methods for integer division (ceil and floor). Because of the repeated division operations in how #4527 tracks compounded interest, those strategies produce measurable skew.

Description of the Design

A rounding strategy without bias toward/away from zero is Round half to even. (It is also free of positivity/negativity bias, which isn't relevant to interest calculation but would have other applications.)

This strategy is also known as bankers' rounding and is the default rounding mode used in IEEE 754 operations for results in binary floating-point formats.

Security Considerations

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Test Plan

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