implemented cached walker

pkg/graph/graph.go

@@ -3,6 +3,7 @@
 package graph
 
 import (
+	"errors"
 	"time"
 )
 
@@ -17,6 +18,11 @@ const (
 	Demotion  int = -1
 )
 
+var (
+	ErrNodeNotFound      = errors.New("node not found")
+	ErrNodeAlreadyExists = errors.New("node already exists")
+)
+
 type ID string
 
 func (id ID) MarshalBinary() ([]byte, error) { return []byte(id), nil }