HDFS should have a NamenodeProtocol to allow secondary namenodes and rebalancing processes to communicate with a primary namenode

For the security purpose, it is nice to have a NamenodeProtocol to allow secondary namenodes and rebalacing processes to communicate with a primary namenode. In the first version the NamenodeProtocol is going to support one RPC:
BlockLocations[] getBlocks(DatanodeID datanode, long size);
This RPC allows a rebalancing process to fetch a partial blocks map from a namenode. It returns a list of blocks on the given datanode and its locations, whose total size is the given size.