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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+1 💚	mvninstall	26m 46s		trunk passed
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+1 💚	spotbugs	1m 42s		trunk passed
+1 💚	shadedclient	22m 45s		branch has no errors when building and testing our client artifacts.
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+1 💚	mvninstall	0m 33s		the patch passed
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+1 💚	javac	0m 37s		the patch passed
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+1 💚	javac	0m 35s		the patch passed
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+1 💚	mvnsite	0m 39s		the patch passed
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+1 💚	javadoc	0m 57s		the patch passed with JDK Private Build-1.8.0_452-8u452-gaus1-0ubuntu120.04-b09
+1 💚	spotbugs	1m 37s		the patch passed
+1 💚	shadedclient	22m 8s		patch has no errors when building and testing our client artifacts.
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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-7862/1/artifact/out/Dockerfile
GITHUB PR	#7862
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux deca3fc16bc1 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-7862/1/testReport/
Max. process+thread count	4349 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
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+0 🆗	codespell	0m 0s		codespell was not available.
+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.
			_ trunk Compile Tests _
+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 💚	spotbugs	1m 41s		trunk passed
+1 💚	shadedclient	22m 4s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 38s		the patch passed
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+1 💚	javac	0m 41s		the patch passed
+1 💚	compile	0m 35s		the patch passed with JDK Private Build-1.8.0_452-8u452-gaus1-0ubuntu120.04-b09
+1 💚	javac	0m 35s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 28s		the patch passed
+1 💚	mvnsite	0m 39s		the patch passed
+1 💚	javadoc	0m 34s		the patch passed with JDK Ubuntu-11.0.27+6-post-Ubuntu-0ubuntu120.04
+1 💚	javadoc	1m 3s		the patch passed with JDK Private Build-1.8.0_452-8u452-gaus1-0ubuntu120.04-b09
+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.
		193m 28s

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
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 49266a6
Default Java	Private Build-1.8.0_452-8u452-gaus1-0ubuntu120.04-b09
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Ubuntu-11.0.27+6-post-Ubuntu-0ubuntu120.04 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_452-8u452-gaus1-0ubuntu120.04-b09
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
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7862/2/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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