core: fix default asset expiration to match C# code

Fixes bogus verification failures for non-native assets.

pkg/core/blockchain.go

@@ -22,6 +22,13 @@ import (
 const (
 	headerBatchCount = 2000
 	version          = "0.0.1"
+
+	// This one comes from C# code and it's different from the constant used
+	// when creating an asset with Neo.Asset.Create interop call. It looks
+	// like 2000000 is coming from the decrementInterval, but C# code doesn't
+	// contain any relationship between the two, so we should follow this
+	// behavior.
+	registeredAssetLifetime = 2 * 2000000
 )
 
 var (
@@ -365,6 +372,7 @@ func (bc *Blockchain) storeBlock(block *Block) error {
 				Precision:  t.Precision,
 				Owner:      t.Owner,
 				Admin:      t.Admin,
+				Expiration: bc.BlockHeight() + registeredAssetLifetime,
 			}
 		case *transaction.IssueTX:
 		case *transaction.ClaimTX: