**How to reproduce**
Clickhouse server 20.13.1.5273
```
CREATE TABLE default.test
(
`key` UInt32,
`ts` DateTime CODEC(DoubleDelta, LZ4),
`db_time` DateTime DEFAULT now() COMMENT 'spout-ignore' CODEC(DoubleDelta, LZ4)
)
ENGINE = MergeTree
PARTITION BY toStartOfTenMinutes(db_time)
ORDER BY (key, ts)
TTL db_time + toIntervalHour(3)
SETTINGS index_granularity = 8192, merge_with_ttl_timeout = 3600, min_rows_for_compact_part = 1000000, min_bytes_for_compact_part = 200000000, in_memory_parts_enable_wal = 0;


INSERT INTO test(key, ts) SELECT number % 1000, now() + intDiv(number,1000) FROM numbers(500);

SELECT * FROM test;
500 rows in set. Elapsed: 0.003 sec.

DETACH TABLE test;

ATTACH TABLE test;

SELECT * FROM test;

0 rows in set. Elapsed: 0.002 sec.
```

**Expected behavior**
Clickhouse would flush parts on disk.
private:
    
    std::mutex mutation_wait_mutex;
    std::condition_variable mutation_wait_event;
    MergeTreeDataSelectExecutor reader;
    MergeTreeDataWriter writer;
    MergeTreeDataMergerMutator merger_mutator;
    std::unique_ptr<MergeTreeDeduplicationLog> deduplication_log;
    
    SimpleIncrement increment;
    
    AtomicStopwatch time_after_previous_cleanup_parts;
    
    AtomicStopwatch time_after_previous_cleanup_temporary_directories;
    
    
    mutable std::mutex currently_processing_in_background_mutex;
    mutable std::condition_variable currently_processing_in_background_condition;
    
    
    DataParts currently_merging_mutating_parts;
    std::map<UInt64, MergeTreeMutationEntry> current_mutations_by_version;