fix memory leak (fix vuejs#88 & vuejs#93)

Author: yyx990803

src/store/api.js

@@ -2,7 +2,9 @@
 import api from 'create-api'
 
 // warm the front page cache every 15 min
-if (api.cachedIds) {
+// make sure to do this only once across all requests
+if (api.onServer && !api.warmCacheStarted) {
+  api.warmCacheStarted = true
   warmCache()
 }
 

src/store/create-api-server.js

@@ -1,5 +1,6 @@
 import Firebase from 'firebase'
 import LRU from 'lru-cache'
+import { fetchItems } from './api'
 
 let api
 const config = {
@@ -12,6 +13,7 @@ if (process.__API__) {
 } else {
   Firebase.initializeApp(config)
   api = process.__API__ = Firebase.database().ref(version)
+  api.onServer = true
 
   // fetched item cache
   api.cachedItems = LRU({