added ability to iterate over all indexes and use index.Iterate() instead of ReadDir() to walk over the graph

Signed-off-by: Roman Strashkin <roman.strashkin@gmail.com>

truncindex/truncindex.go

@@ -108,3 +108,13 @@ func (idx *TruncIndex) Get(s string) (string, error) {
 	}
 	return "", fmt.Errorf("no such id: %s", s)
 }
+
+// Iterates over all stored IDs, and passes each of them to the given handler
+func (idx *TruncIndex) Iterate(handler func(id string)) {
+	idx.RLock()
+	defer idx.RUnlock()
+	idx.trie.Visit(func(prefix patricia.Prefix, item patricia.Item) error {
+		handler(string(prefix))
+		return nil
+	})
+}