feat: improve invariant checks and tests

[dongsam]

Summary

Improve and add invariants for farming module.

Proposal

Let's first come up with what invariant checks the farming module needs and implement those afterwards.

Tasks

• Add test cases to cover those invariants
• Add, Improve invariants in farming module

References

• Cosmos SDK Invariants Docs
• Example: gov module invariants
• Example: staking module invariants
• Example: farming module invariants

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[hallazzang]

I propose these invariants for the farming module:

• The balance of StakingReserveAcc must be greater or equal than coins in all Stakings and QueuedStakings.
• The balance of RewardsReserveAcc must be greater or equal than total rewards calculated by WithdrawRewards.
• The balance of RewardsReserveAcc must be greater or equal than coins in all OutstandingRewards.
• Plans that should have been terminated should be marked as terminated.
• HistoricalRewards must not have negative CumulativeUnitRewards.
• OutstandingRewards must not have negative Rewards.
• TotalStakings must not have negative Amount.
• Staking and QueuedStaking must not have negative Amount.

[hallazzang]

Additional invariants:

• Reference count must be consistent(if feat: Prune states when data is no longer needed #142 were resolved).