fix: improve budget validation logic and tests, spec doc

[dongsam]

Description

closes: #109

Tasks

• [5] enhance the logic to only consider overlapping budgets when calculating the total rate for each time slot Incorrect budget validation may reject valid overlapping budget plans #109
• [7] update spec doc more precisely
• [18] add a check to the Validate function that ensures that source and destination addresses are different.
• [12] simplify budget_collected event
• refactor budget.Collectible to use DateRangeIncludes
• refactor and add test cases for budgets
• add omitted ValidateEpochBlocks on params
• fix budget collection logic more conservatively and safely
• fix simulation to cover edge cases
• fix to get balance only spendable on budget source

References

• Audit Report Budget and Farming Cosmos SDK Modules v0.1

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• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
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[jaybxyz]

I think all the tasks are covered well. Great work! LGTM

[hallazzang]

LGTM overall. There's just a minor suggestion.