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.
{
    setThreadName(thread_name.c_str());
    attachToThreadGroup();
    SCOPE_EXIT({ CurrentThread::detachQueryIfNotDetached(); });
    if (auto * memory_tracker = CurrentThread::getMemoryTracker())
        memory_tracker->setMetric(memory_metric);
    while (!shutdown)
    {
        if (Poco::AutoPtr<Poco::Notification> notification = queue.waitDequeueNotification())
        {
            TaskNotification & task_notification = static_cast<TaskNotification &>(*notification);
            task_notification.execute();
        }
    }
}
void BackgroundSchedulePool::delayExecutionThreadFunction()
{
    setThreadName((thread_name + "/D").c_str());
    attachToThreadGroup();
    SCOPE_EXIT({ CurrentThread::detachQueryIfNotDetached(); });
    while (!shutdown)
    {
        TaskInfoPtr task;
        bool found = false;
        {
            std::unique_lock lock(delayed_tasks_mutex);
            while (!shutdown)
            {
                Poco::Timestamp min_time;
                if (!delayed_tasks.empty())