x/mint: refactor, document and unit test keeper's DistributeMintedCoin

[p0mvn]

Background

This is a follow-up to #1839

DistributeMintedCoin does too many things and violates the separation of concerns. It should be refactored to focus on one thing and potentially call other methods.

Suggested Design

• Distributing developer rewards has its own method that is unit tested
• Distributing staking incentives, pool incentives and community pool incentives can be refactored into a single reusable method and unit tested

Acceptance Criteria

• the suggested refactor is complete to extract 2 functions:

1. For developer rewards distribution
2. Reusable function for distributing staking, pool incentives and community pool rewards.

• the newly extracted methods are fully covered by unit tests
• other tests continue to pass

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[p0mvn]

Marking this as blocked by #1915.

This is better be done after the existing tests are understood and refactored