Enhancements for background merges

[nikitamikhaylov]

Changelog category (leave one):

• Improvement

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

Introduced settings:

• merge_max_block_size_bytes to limit the amount of memory used for background operations.
• vertical_merge_algorithm_min_bytes_to_activate to add another condition to activate vertical merges.

[robot-ch-test-poll3]

This is an automated comment for commit ab35601 with description of existing statuses. It's updated for the latest CI running
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The overall status of the commit is 🟡 pending

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[nikitamikhaylov]

Something strange with cache #49368

[nikitamikhaylov]

Disabled the test with S3 storage, because we really eat a lot of memory there.