Add endBlock into epoch snapshot

Author: Mike-CZ

contracts/sfc/SFC.sol

@@ -369,6 +369,7 @@ contract SFC is SFCBase, Version {
 
         currentSealedEpoch = currentEpoch();
         snapshot.endTime = _now();
+        snapshot.endBlock = block.number;
         snapshot.baseRewardPerSecond = c.baseRewardPerSecond();
         snapshot.totalSupply = totalSupply;
     }

contracts/sfc/SFCState.sol

@@ -71,6 +71,7 @@ contract SFCState is Initializable, Ownable {
         mapping(uint256 => uint256) offlineBlocks;
         uint256[] validatorIDs;
         uint256 endTime;
+        uint256 endBlock;
         uint256 epochFee;
         uint256 totalBaseRewardWeight;
         uint256 totalTxRewardWeight;