Do Device object persistent on receiving reports.
Workable create_file implementation, recv MetaStoreClient's request and write objects to database through JDO.
Add DM Clean/Rep thread to handle file close/rm_physical.
Interact with DService, file replication and deletion is working now
Fix NullPointer bugs in node invalidation; and add MDataCenter table.
Fix concurrent bugs in DiskManager.
README.test.by branch test2. liulichao