Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

);
                if (thread_group)
                    CurrentThread::attachTo(thread_group);
                remove_part(*part);
            });
        }
        pool.wait();
    }
    else
    {
        for (const DataPartPtr & part : parts)
        {
            remove_part(*part);
        }
    }
}
void StorageReplicatedMergeTree::clearOldPartsAndRemoveFromZK()
{
    auto table_lock = lockForShare(
            RWLockImpl::NO_QUERY, getSettings()->lock_acquire_timeout_for_background_operations);
    auto zookeeper = getZooKeeper();
    DataPartsVector parts = grabOldParts();
    if (parts.empty())
        return;
    DataPartsVector parts_to_delete_only_from_filesystem;    
    DataPartsVector parts_to_delete_completely;              
    DataPartsVector parts_to_retry_deletion;                 
    DataPartsVector parts_to_remove_from_filesystem;         
    for (const auto & part : parts)
    {