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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+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
+1 💚	test4tests	0m 0s		The patch appears to include 1 new or modified test files.
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+1 💚	mvninstall	25m 59s		trunk passed
+1 💚	compile	0m 44s		trunk passed with JDK Ubuntu-11.0.27+6-post-Ubuntu-0ubuntu120.04
+1 💚	compile	0m 39s		trunk passed with JDK Private Build-1.8.0_452-8u452-gaus1-0ubuntu120.04-b09
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+1 💚	javac	0m 35s		the patch passed
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+1 💚	spotbugs	1m 42s		the patch passed
+1 💚	shadedclient	22m 18s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	109m 57s		hadoop-hdfs in the patch passed.
+1 💚	asflicense	0m 30s		The patch does not generate ASF License warnings.
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Subsystem	Report/Notes
Docker	ClientAPI=1.51 ServerAPI=1.51 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7862/2/artifact/out/Dockerfile
GITHUB PR	#7862
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux c407aa9d25a0 5.15.0-143-generic #153-Ubuntu SMP Fri Jun 13 19:10:45 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
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Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7862/2/testReport/
Max. process+thread count	3930 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
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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.