(Feature) BlockReward contract

Relates to poanetwork/RFC#14

contracts/BlockReward.sol

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+pragma solidity ^0.4.23;
+
+import "./interfaces/IBlockReward.sol";
+import "./interfaces/IKeysManager.sol";
+import "./interfaces/IProxyStorage.sol";
+
+
+contract BlockReward is IBlockReward {
+    address constant SYSTEM_ADDRESS = 0xffffFFFfFFffffffffffffffFfFFFfffFFFfFFfE;
+
+    IProxyStorage public proxyStorage;
+    address public emissionFunds;
+    uint256 public blockRewardAmount;
+    uint256 public emissionFundsAmount;
+
+    modifier onlySystem {
+        require(msg.sender == SYSTEM_ADDRESS);
+        _;
+    }
+
+    constructor(
+        address _proxyStorage,
+        address _emissionFunds,
+        uint256 _blockRewardAmount,
+        uint256 _emissionFundsAmount
+    ) public {
+        require(_proxyStorage != address(0));
+        require(_blockRewardAmount != 0);
+        proxyStorage = IProxyStorage(_proxyStorage);
+        emissionFunds = _emissionFunds;
+        blockRewardAmount = _blockRewardAmount;
+        emissionFundsAmount = _emissionFundsAmount;
+    }
+
+    function reward(address[] benefactors, uint16[] kind)
+        external
+        onlySystem
+        returns (address[], uint256[])
+    {
+        require(benefactors.length == kind.length);
+        require(benefactors.length == 1);
+        require(kind[0] == 0);
+
+        address miningKey = benefactors[0];
+        address payoutKey = _getPayoutByMining(miningKey);
+
+        require(payoutKey != address(0));
+
+        uint256 receiversLength = 2;
+
+        if (emissionFunds == address(0) || emissionFundsAmount == 0) {
+            receiversLength = 1;
+        }
+
+        address[] memory receivers = new address[](receiversLength);
+        uint256[] memory rewards = new uint256[](receiversLength);
+
+        receivers[0] = payoutKey;
+        rewards[0] = blockRewardAmount;
+
+        if (receiversLength == 2) {
+            receivers[1] = emissionFunds;
+            rewards[1] = emissionFundsAmount;
+        }
+
+        return (receivers, rewards);
+    }
+
+    function _getPayoutByMining(address _miningKey)
+    	private
+    	view
+    	returns (address)
+    {
+    	IKeysManager keysManager = IKeysManager(proxyStorage.getKeysManager());
+    	return keysManager.getPayoutByMining(_miningKey);
+    }
+}

contracts/interfaces/IBlockReward.sol

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+pragma solidity ^0.4.23;
+
+
+interface IBlockReward {
+    // Produce rewards for the given benefactors, with corresponding reward codes.
+    // Only callable by `SYSTEM_ADDRESS`
+    function reward(address[], uint16[]) external returns (address[], uint256[]);
+}