fix: refactor RewardsReserveAcc to be derived by deriveAddress

[dongsam]

Description

closes: #XXXX

Tasks

• refactor RewardsReserveAcc to be derived by deriveAddress ( address not changed EFD5E11CC3F9425D8B333D67F56EDB47E6994C7548C6061C455261C9BAE21E80)
• remove caution comment on readme

References

• Split up reserve accounts per staking coin denom #220

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