params: activate Istanbul on Ropsten and Görli

Author: soc1c

params/config.go

@@ -103,7 +103,7 @@ var (
 		ByzantiumBlock:      big.NewInt(1700000),
 		ConstantinopleBlock: big.NewInt(4230000),
 		PetersburgBlock:     big.NewInt(4939394),
-		IstanbulBlock:       nil,
+		IstanbulBlock:       big.NewInt(6485846),
 		Ethash:              new(EthashConfig),
 	}
 
@@ -180,7 +180,7 @@ var (
 		ByzantiumBlock:      big.NewInt(0),
 		ConstantinopleBlock: big.NewInt(0),
 		PetersburgBlock:     big.NewInt(0),
-		IstanbulBlock:       nil,
+		IstanbulBlock:       big.NewInt(1561651),
 		Clique: &CliqueConfig{
 			Period: 15,
 			Epoch:  30000,