fixed late-join sync issue & use leveldb only in node environment

src/SpecHelper.js

@@ -11,7 +11,9 @@ require('../../y-memory/src/Memory.js')(Y)
 require('../../y-array/src/Array.js')(Y)
 require('../../y-map/src/Map.js')(Y)
 require('../../y-indexeddb/src/IndexedDB.js')(Y)
-require('../../y-leveldb/src/LevelDB.js')(Y)
+if (typeof window === 'undefined') {
+  require('../../y-leveldb/src/LevelDB.js')(Y)
+}
 module.exports = Y
 
 var g

src/Transaction.js

@@ -997,6 +997,7 @@ module.exports = function (Y/* :any */) {
           if (firstMissing != null) {
             // update startPos
             startPos = firstMissing.id[1]
+            startSS[user] = startPos
           }
         }
         yield* this.os.iterate(this, [user, startPos], [user, Number.MAX_VALUE], function * (op) {