new_sync branch, enhance some codes for better performance.

1)   org.apache.iotdb.db.newsync.sender.service.SenderService#getInstance
     This singleton class  SenderService's performance is not good. suggest to use Holder singleton mode.

2)  server/src/main/java/org/apache/iotdb/db/newsync/receiver/ReceiverService.java  line 49

import static org.apache.iotdb.db.conf.IoTDBConstant.*; 

    remove the wildcard char.

3) org.apache.iotdb.db.newsync.pipedata.SchemaPipeData#sendToTransport
     remove System.out.println()