public static SyncDataNodeClientPool getInstance() {
    return ClientPoolHolder.INSTANCE;
  }
change a region leader from the datanode to other datanode, other datanode should be in same
raft group.

@param regionId the region which will changer leader
@param dataNode data node server, change regions leader from it
@param newLeaderNode target data node, change regions leader to it
@return the result of the change leader operation
iotdb-core/confignode/src/main/java/org/apache/iotdb/confignode/client/sync/SyncDataNodeClientPool.java