HDFS-17818. Fix serial fsimage transfer during checkpoint with multiple namenodes

[lfxy]

In our cluster, each namespace has four NameNodes: one active, one standby, and two observers. When the standby NameNode performs a checkpoint, it transfer the fsimage to the other three NameNodes. However, we found that these transfer are performed serially.
The reason is that the corePoolSize in ThreadPoolExecutor is 0, and the transfer task does not fill the LinkedBlockingQueue, resulting in only one thread transfer the fsimage at a time. This greatly increases the checkpoint time.
 ExecutorService executor = new ThreadPoolExecutor(0, activeNNAddresses.size(), 100,         TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(activeNNAddresses.size()),         uploadThreadFactory);

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[lfxy]

@Hexiaoqiao @ayushtkn @tomscut Do you think that uploading fsimage in checkpoint with observer namenode should be changed from serial to parallel?
In our production cluster, there are one active, one standby, and two oberver namenodes. The serial uploading takes 40 minutes longer than the parallel operation.

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