flow: add initial flow dag

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

flow/dag.go

@@ -0,0 +1,21 @@
+package flow
+
+type node struct {
+	name string
+}
+
+func (n *node) ID() string {
+	return n.name
+}
+
+func (n *node) Name() string {
+	return n.name
+}
+
+func (n *node) String() string {
+	return n.name
+}
+
+func (n *node) Hashcode() interface{} {
+	return n.name
+}