Inconsistent baseFee check logic in code

[yihuang]

System info: ethermint main

Following the discussion here: #671 (comment), @fedekunze

Steps to reproduce:

Currently, different places have different logic to check base fee:

• https://github.com/tharsis/ethermint/blob/main/x/evm/keeper/state_transition.go#L36
• https://github.com/tharsis/ethermint/blob/main/app/ante/eth.go#L366

Better to encapsulate a common logic to do this.

Expected behavior: consistent way to check base fee.

Actual behavior: inconsistency

Additional info:

[yihuang]

if not london_hardfork {
    # reject DynamicFeeTx
    # no `baseFeePerGas` field in block response
    # baseFee = nil
} else {
    # allow DynamicFeeTx
    # add `baseFeePerGas` field in block response
    if feemarketParams.NoBaseFee or height < feemarketParams.EnableHeight {
        # baseFee = 0
    } else {
        # init baseFee to initBaseFee and adjust in later blocks
    }
}

ref: https://github.com/ethereum/go-ethereum/blob/master/internal/ethapi/api.go#L1211

[thomas-nguy]

It looks reasonable to me.

For the case NoBaseFee or height < feemarketParams.EnableHeigh
The cost of the transaction will be only the gas-cap which will be the local value per validator (min-gas-price). In that case we are also consistent with previous behavior when london hard fork is not activated