Validate CX receipt root from CrossTxEpoch

Previously, validation started at CrossTxEpoch+1 and CX receipts roots
were unverified in CrossTxEpoch, creating a security hole.

core/block_validator.go

@@ -100,7 +100,7 @@ func (v *BlockValidator) ValidateState(block, parent *types.Block, statedb *stat
 		return fmt.Errorf("invalid receipt root hash (remote: %x local: %x)", header.ReceiptHash(), receiptSha)
 	}
 
-	if v.config.AcceptsCrossTx(block.Epoch()) {
+	if v.config.HasCrossTxFields(block.Epoch()) {
 		cxsSha := types.DeriveMultipleShardsSha(cxReceipts)
 		if cxsSha != header.OutgoingReceiptHash() {
 			return fmt.Errorf("invalid cross shard receipt root hash (remote: %x local: %x)", header.OutgoingReceiptHash(), cxsSha)