minimal-gas-prices and baseFeePerGas conflicts

[yihuang]

Discussed in #830

^{Originally posted by yihuang December 13, 2021}
Currently, when the feemarket module is enabled, for eth transactions we have an extra requirement:

effectiveGasPrice >= minimal-gas-prices
where effectiveGasPrice = min(baseFeePerGas + tipCap, feeCap) for `DynamicFeeTx`
      effectiveGasPrice = gasPrice for `LegacyTx`

Usually, clients pick zero or constant tipCap for dynamic fee tx, and baseFeePerGas keep dropping when blocks are sparse, when baseFeePerGas drop to a point where effectiveGasPrice < minimal-gas-prices, the dynamic fee tx will fail, and client have to re-calculate a tipCap based on current minimal-gas-prices and baseFeePerGas. So basically the client logic will be different from go-ethereum.

A simple solution is don't check minimal-gas-prices at all for eth tx when feemarket is enabled. And for legacy tx, rather than check tx.gasPrice >= minimal-gas-prices, we check tx.gasPrice >= baseFeePerGas.

And considering the native cosmos tx, we shouldn't set min-gas-price too low, a reasonable one might be set min-gas-price = initial base fee.