Implement list and fix test

cache.go

@@ -1,7 +1,6 @@
 package storage
 
 import (
-	"errors"
 	"os"
 	"path"
 	"time"
@@ -27,7 +26,18 @@ func NewCache(path ...string) (*Cache, error) {
 }
 
 func (c *Cache) List(ns []string, limits ...string) ([]string, error) {
-	return nil, errors.New("not impl")
+	namespace := resolveNamespace(ns)
+	limits = resolveLimits(limits)
+	limits[0] = path.Join(namespace, limits[0])
+	limits[1] = path.Join(namespace, limits[1])
+	m := c.db.Items()
+	keys := []string{}
+	for k := range m {
+		if k >= limits[0] && k <= limits[1] {
+			keys = append(keys, k)
+		}
+	}
+	return keys, nil
 }
 
 func (c *Cache) Get(key string, ns ...string) ([]byte, error) {