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.
{
    Poco::Timestamp current_time;
    task->allow_execute_in_parallel = allow_execute_in_parallel;
    {
        std::unique_lock lock(tasks_mutex);
        task->iterator = tasks.emplace(current_time, task);
        wake_event.notify_all();
    }
}
BackgroundProcessingPool::TaskHandle BackgroundProcessingPool::addTask(const Task & task)
{
    TaskHandle res = createTask(task);
    startTask(res);
    return res;
}
void BackgroundProcessingPool::removeTask(const TaskHandle & task)
{
    if (task->removed.exchange(true))
        return;
    
    {
        std::unique_lock wlock(task->rwlock);
    }
    {
        std::unique_lock lock(tasks_mutex);
        tasks.erase(task->iterator);
        
    }
}
BackgroundProcessingPool::~BackgroundProcessingPool()