25.8 Altinity backport of #91574: Add cleanup thread to MergeTree to avoid cleanup starvation

[ilejn]

25.8 Antalya backport of ClickHouse#91574: Add cleanup thread to MergeTree to avoid cleanup starvation

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Add a dedicated cleanup thread for MergeTree to prevent cleanup delays under heavy merge load. This resolves ClickHouse#86181.

The MergeTree cleanup logic was tightly coupled with the merge scheduler, which caused cleanup tasks to be delayed or completely blocked when the table was under heavy merge load. ReplicatedMergeTree already had a separate cleanup thread, so this change aligns MergeTree with that design. @azat You might be interested in this one.
(ClickHouse#91574 by amosbird)

Documentation entry for user-facing changes

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CI/CD Options

Exclude tests:

• Fast test
• Integration Tests
• Stateless tests
• Stateful tests
• Performance tests
• Aarch64 tests
• All with ASAN
• All with TSAN
• All with MSAN
• All with UBSAN
• All with Coverage
• All Regression
• Disable CI Cache

Regression jobs to run:

• Fast suites (mostly <1h)
• Aggregate Functions (2h)
• Alter (1.5h)
• Benchmark (30m)
• ClickHouse Keeper (1h)
• Iceberg (2h)
• LDAP (1h)
• OAuth (5m)
• Parquet (1.5h)
• RBAC (1.5h)
• SSL Server (1h)
• S3 (2h)
• S3 Export (2h)
• Swarms (30m)
• Tiered Storage (2h)

[ilejn]

Does not require review, close to #1589

[github-actions]

Workflow [PR], commit [81ab4c2]