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.
{
    return TaskHolder(std::make_shared<TaskInfo>(*this, name, function));
}
void BackgroundSchedulePool::scheduleDelayedTask(const TaskInfoPtr & task, size_t ms, std::lock_guard<std::mutex> & )
{
    Poco::Timestamp current_time;
    {
        std::lock_guard lock(delayed_tasks_mutex);
        if (task->delayed)
            delayed_tasks.erase(task->iterator);
        task->iterator = delayed_tasks.emplace(current_time + (ms * 1000), task);
        task->delayed = true;
    }
    wakeup_cond.notify_all();
}
void BackgroundSchedulePool::cancelDelayedTask(const TaskInfoPtr & task, std::lock_guard<std::mutex> & )