R4R: Refactor Gas/Fee Model

[alexanderbez]

• Remove minimum-fees in favor of minimum-gas-prices
• Update ante handler to check gasPrices >= minGasPrices
• Since gasPrices are of type DecCoins, I had to add a fair amount of functionality to Dec and DecCoin/s.
• Remove OR semantics when determining if min fees are provided in a tx. Instead, we revert back to AND semantics (per convo w/ @jaekwon and @zmanian).

Note: I'll be happy to move all the decimal related changes to a separate PR if that will help ease review.

closes: #3248

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• Wrote tests

• Updated relevant documentation (docs/)

• Added entries in PENDING.md with issue #

• rereviewed Files changed in the github PR explorer

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[hendrikhofstadt]

Now that we're using gasPrice as metric we could open a follow-up issue with optional mempool sorting by fee.

[alexanderbez]

@cwgoes @jaekwon this should be r4r again.

[jackzampolin]

Thanks for addressing those comments quickly @alexanderbez. I reviewed with a focus on docs and tests, and this PR is looking ready to merge!