Add Clear methods to lru cache

lru/lru.go

@@ -119,3 +119,15 @@ func (c *Cache) Len() int {
 	}
 	return c.ll.Len()
 }
+
+// Clear purges all stored items from the cache.
+func (c *Cache) Clear() {
+	if c.OnEvicted != nil {
+		for _, e := range c.cache {
+			kv := e.Value.(*entry)
+			c.OnEvicted(kv.key, kv.value)
+		}
+	}
+	c.ll = nil
+	c.cache = nil
+}