Fix maxFeePerGas < maxPriorityFeePerGas transaction issue

[shawntobin]

resolves #1136

Replaced legacy gas calculation that sometimes caused maxFeePerGas to be less than maxPriorityFeePerGas.

Previous code used web3.eth.getGasPrice() to calculate maxFeePerGas, which isn't suitable anymore for EIP-1559 transactions. Now fetching 'baseFeePerGas' instead.

[master-malwyl]

One minor suggestion: we might want to add an explicit error case if both 'pending' and 'latest' blocks fail to return baseFeePerGas:

let block = await web3.eth.getBlock('pending');

// in case pending block doesn't return anything
if (!block || !block.baseFeePerGas) {
  block = await web3.eth.getBlock('latest');
  if (!block || !block.baseFeePerGas) {
    throw new Error('Unable to fetch baseFeePerGas from network');
  }
}

This would make debugging easier if we ever hit an edge case where the network isn't returning proper EIP-1559 data.

Otherwise, the changes look good and should fix the issues reported in #1136.