Make empty cache detection a bit more aggressive to account for cache flush and data insert happening concurrently

Author: sulkaharo

lib/server/cache.js

@@ -53,7 +53,7 @@ function cache (env, ctx) {
   }
 
   data.isEmpty = (datatype) => {
-    return data[datatype].length == 0;
+    return data[datatype].length < 20;
   }
 
   data.getData = (datatype) => {