1. change default thread pool core size from 2 to 0
2. support refresh when state is StateLoadCompleted
3. bug fix: duplicate listener after listen data with same listener more than once
1. add support for group pre-load task with GroupedDataLoader and GroupedDataListener
1. add javadoc for PreLoader
2. don`t accept listeners after pre-laod task has been destroyed
add support for remove DataListener dynamically