refactor: Reduce parallelism when vacuuming dropped tables

[dantengsky]

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Summary

Limit parallelism to a maximum of 3 when vacuuming dropped tables.

• for each vacuum task, the tables passed to it will be processed sequentially
• while removing one table's data, at most 1000 objects will be deleted (in batch)
• let's assume that the rate limit is 3500 (individual) objects per second:
  A parallelism degree of up to 3 appears to be safe.

fixes: #16311

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - tweak vacuuming dropped table parallelism

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is