When I queried `system.metrics` table, I found some negative values which hint to some potential internal overflows.

```
SELECT *
FROM system.metrics
FORMAT Vertical

...

Row 31:
───────
metric:      MemoryTrackingInBackgroundProcessingPool
value:       -580863171
description: Total amount of memory (bytes) allocated in background processing pool (that is dedicated for backround merges, mutations and fetches). Note that this value may include a drift when the memory was allocated in a context of background processing pool and freed in other context or vice-versa. This happens naturally due to caches for tables indexes and doesn't indicate memory leaks.

Row 32:
───────
metric:      MemoryTrackingInBackgroundMoveProcessingPool
value:       0
description: Total amount of memory (bytes) allocated in background processing pool (that is dedicated for backround moves). Note that this value may include a drift when the memory was allocated in a context of background processing pool and freed in other context or vice-versa. This happens naturally due to caches for tables indexes and doesn't indicate memory leaks.

Row 33:
───────
metric:      MemoryTrackingInBackgroundSchedulePool
value:       -4461308831
description: Total amount of memory (bytes) allocated in background schedule pool (that is dedicated for bookkeeping tasks of Replicated tables).

...

56 rows in set. Elapsed: 0.003 sec.
```

Server version: 20.8.2.3.
const auto current_cycle = close_cycle;
        ++close_cycle;
        close_cycle_time = now + close_interval;
        if (close_times.empty())
            return close_interval;
        auto & sessions_to_close = close_times.front();
        for (const auto & key : sessions_to_close)
        {
            const auto session = sessions.find(key);
            if (session != sessions.end() && session->second->close_cycle <= current_cycle)
            {
                if (!session->second.unique())
                {
                    
                    session->second->timeout = std::chrono::steady_clock::duration{0};
                    scheduleCloseSession(*session->second, lock);
                }
                else
                    sessions.erase(session);
            }
        }
        close_times.pop_front();
        return close_interval;
    }
    std::mutex mutex;
    std::condition_variable cond;
    std::atomic<bool> quit{false};
    ThreadFromGlobalPool thread{&NamedSessions::cleanThread, this};
};
void NamedSession::release()