Replace internal usages of 'master' terminology in server directory

[tlfeng]

Description

• Replace the non-inclusive terminology "master" with "cluster manager" in code comments, internal variable/method/class names, where the backwards compatibility is not impacted, in server directory.
• The PR is split from Replace non-inclusive terminology 'master' in code comments and internal variable/method/class names #2150

Replacement rules:

• master -> cluster_manager or clusterManager (variable name) or cluster-manager (code comment)
• Master -> ClusterManager

Issues Resolved

A part of #1548

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[opensearch-ci-bot]

❌   Gradle Check failure 6813827c36dbe7ed66b5b5262e263438daca44e9
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Reports 3541

[opensearch-ci-bot]

❌   Gradle Check failure 7dbf60b245114a422be2097514b6fba8f21792cc
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[opensearch-ci-bot]

❌   Gradle Check failure b08c5b0e929a704fbd1f74f8d2a7f327d107b020
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[tlfeng]

I will split the PR further by sub-directory to control the lines of code change. (One spit PR is #2519)

[opensearch-ci-bot]

❌   Gradle Check failure 74b3065
Log 3547

Reports 3547