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.
{
    std::lock_guard lock_exec(exec_mutex);
    std::lock_guard lock_schedule(schedule_mutex);
    if (deactivated)
        return;
    deactivated = true;
    scheduled = false;
    if (delayed)
        pool.cancelDelayedTask(shared_from_this(), lock_schedule);
}
void BackgroundSchedulePoolTaskInfo::activate()
{
    std::lock_guard lock(schedule_mutex);
    deactivated = false;
}
bool BackgroundSchedulePoolTaskInfo::activateAndSchedule()
{
    std::lock_guard lock(schedule_mutex);
    deactivated = false;
    if (scheduled)
        return false;
    scheduleImpl(lock);
    return true;
}
void BackgroundSchedulePoolTaskInfo::execute()