[#49] Support contract migration

At initialization contract saves master script hash
that allows to re-initialize or migrate contract.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

common/update.go

@@ -0,0 +1,22 @@
+package common
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/interop"
+	"github.com/nspcc-dev/neo-go/pkg/interop/runtime"
+	"github.com/nspcc-dev/neo-go/pkg/interop/storage"
+)
+
+const OwnerKey = "contractOwner"
+
+// HasUpdateAccess returns true if contract can be initialized, re-initialized
+// or migrated.
+func HasUpdateAccess(ctx storage.Context) bool {
+	data := storage.Get(ctx, OwnerKey)
+	if data == nil { // contract has not been initialized yet, return true
+		return true
+	}
+
+	owner := data.(interop.Hash160)
+
+	return runtime.CheckWitness(owner)
+}