Add fee manager for Native-to-ERC that collects fees on home network

[patitonar]

Closes #138

I had to make some changes on HomeBridgeNativeToErc.sol to make it work with the two fee manager approaches on this bridge mode.

Also I refactored how the .env variables HOME_REWARDABLE and FOREIGN_REWARDABLE works to be able to deploy the correct contracts on the deploy script.

After we agree on the changes, I'll update the gas consumption document as part of this PR.

[akolotov]

I think we can simplify things a bit.

If we look at the Home Bridge functionality from the Fee Management point of view it could be as following:

1. the fallback function

• change the value to transfer (if the Home fee is not zero. The Fee Manager is responsible for both directions)
• keep the value to transfer (if the Home fee is zero. There is no fee manager or it is responsible for one direction)

2. onSignaturesCollected

• recover fee from the message and distribute it among the validators (if the Home fee is not zero. The Fee Manager is responsible for both directions)
• nothing special (if the Home fee is zero. There is no fee manager or it is responsible for one direction)
  The same is applicable for the Foreign Bridge functionality:
• onExecuteMessage
  • change the value received from the message, send the corresponding value of assets, distribute fee among the validators (if the Home fee is not zero. The Fee Manager is responsible for one directions)
  • keep the value received from the message unchanged and send the corresponding value of assets (if the Home fee is zero. There is no fee manager)

So, we don't need to introduce onRequestForSignature or onSignaturesCollected in the Fee Manager contract. We can implement similar we already done for the erc-to-native mode:

fallback, the HomeBridge contract

  uint256 valueToTransfer = msg.value;
  address feeManager = feeManagerContract();
  if (feeManager != address(0)) {
      uint256 fee = calculateFee(valueToTransfer, false, feeManager, HOME_FEE);
      valueToTransfer = valueToTransfer.sub(fee);
  }
  emit UserRequestForSignature(msg.sender, valueToTransfer);

It will work for both types of Fee Managers since the Home Fee is zero when there is no fee manager or it is responsible for one direction (called for onExecuteAffirmation only)

onSignaturesCollected, the HomeBridge contract

  address feeManager = feeManagerContract();
  if (feeManager != address(0)) {
      address recipient;
      uint256 amount;
      bytes32 txHash;
      address contractAddress;
      (recipient, amount, txHash, contractAddress) = Message.parseMessage(_message);
      uint256 fee = calculateFee(amount, true, feeManager, HOME_FEE);
      if (fee != 0) {
          distributeFeeFromSignatures(fee, feeManager);
      }
  }

By adding if (fee != 0) it will work for both types of Fee Managers since the Home Fee is zero when there is no fee manager or it is responsible for one direction

onExecuteMessage, the ForeignBridge contract

  uint256 valueToMint = _amount;
  address feeManager = feeManagerContract();
  if (feeManager != address(0)) {
      uint256 fee = calculateFee(valueToMint, false, feeManager, HOME_FEE);
      if (fee != 0) {
          distributeFeeFromSignatures(fee, feeManager);
          valueToMint = valueToMint.sub(fee);
      }
  }

By adding if (fee != 0) it will work in both scenarios as well. In the scenario when there is no Fee Manager the fee will be calculated as zero.

[patitonar]

Thanks for the suggestions, I applied the changes. If no more changes are needed, I'll update gas consumption document

[akolotov]

Thanks for bringing this PR up to date! Let's merge it