HDFS-17106. NameNode can timeout during initialization with dfs.datanode.volumes.replica-add.threadpool.size being 0

[teamconfx]

Description of PR

When dfs.datanode.volumes.replica-add.threadpool.size is 0, HDFS cluster is never able to start and gets timed out eventually.

To reproduce:

1. set dfs.datanode.volumes.replica-add.threadpool.size to 0
2. run mvn surefire:test -Dtest=org.apache.hadoop.hdfs.server.namenode.metrics.TestNameNodeMetrics#testExcessBlocks

This PR improves the description of replica-add.threadpool.size in hdfs-default.xml by sepecifying that it should be positive; the PR also checks whether the value is positive before it is used to initialize addReplicaThreadPool.

How was this patch tested?

Unit test

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 45s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+0 🆗	xmllint	0m 0s		xmllint was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
-1 ❌	test4tests	0m 0s		The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
			_ trunk Compile Tests _
+1 💚	mvninstall	46m 49s		trunk passed
+1 💚	compile	1m 21s		trunk passed with JDK Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu120.04
+1 💚	compile	1m 11s		trunk passed with JDK Private Build-1.8.0_382-8u382-ga-1~20.04.1-b05
+1 💚	checkstyle	1m 13s		trunk passed
+1 💚	mvnsite	1m 22s		trunk passed
+1 💚	javadoc	1m 8s		trunk passed with JDK Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu120.04
+1 💚	javadoc	1m 34s		trunk passed with JDK Private Build-1.8.0_382-8u382-ga-1~20.04.1-b05
+1 💚	spotbugs	3m 22s		trunk passed
+1 💚	shadedclient	39m 32s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 10s		the patch passed
+1 💚	compile	1m 15s		the patch passed with JDK Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu120.04
+1 💚	javac	1m 15s		the patch passed
+1 💚	compile	1m 8s		the patch passed with JDK Private Build-1.8.0_382-8u382-ga-1~20.04.1-b05
+1 💚	javac	1m 8s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	1m 1s		the patch passed
+1 💚	mvnsite	1m 13s		the patch passed
+1 💚	javadoc	0m 55s		the patch passed with JDK Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu120.04
+1 💚	javadoc	1m 26s		the patch passed with JDK Private Build-1.8.0_382-8u382-ga-1~20.04.1-b05
+1 💚	spotbugs	3m 19s		the patch passed
+1 💚	shadedclient	39m 16s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
-1 ❌	unit	252m 46s	/patch-unit-hadoop-hdfs-project_hadoop-hdfs.txt	hadoop-hdfs in the patch passed.
+1 💚	asflicense	0m 46s		The patch does not generate ASF License warnings.
		401m 58s

Reason	Tests
Failed junit tests	hadoop.hdfs.server.datanode.TestDirectoryScanner

Subsystem	Report/Notes
Docker	ClientAPI=1.43 ServerAPI=1.43 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-6090/1/artifact/out/Dockerfile
GITHUB PR	#6090
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets xmllint
uname	Linux edff6a362be4 5.15.0-78-generic #85-Ubuntu SMP Fri Jul 7 15:25:09 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / cf15f6c
Default Java	Private Build-1.8.0_382-8u382-ga-1~20.04.1-b05
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Ubuntu-11.0.20+8-post-Ubuntu-1ubuntu120.04 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_382-8u382-ga-1~20.04.1-b05
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-6090/1/testReport/
Max. process+thread count	2089 (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-6090/1/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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