Add MintPoolAddress on mint params to separate inflation with tx fee

[dongsam]

Summary

Add params.MintPoolAddress on the mint module to customize and separate collected tx fee from FeeCollector

Proposal

mint_pool_address defines the address where inflation will be minted, The default is FeeCollector It's the same value that's currently hardcoded, but if it set to FeeCollector, minted inflation could be mixed together with collected tx fees. Therefore, it is recommended to specify a separate address depending on the chain

Tasks

• [x] add params.MintPoolAddress
• [x] update ConsensusVersion of mint module 1 -> 2
• [x] add v2 upgrade handler ( state, migration )
• [x] update docs ( spec, swagger )

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