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. - 

all_diff_disk_names.erase(disk->getName());
                    for (const String & disk_name : all_diff_disk_names)
                    {
                        auto disk = new_storage_policy->getDiskByName(disk_name);
                        if (disk->exists(relative_data_path))
                            throw Exception("New storage policy contain disks which already contain data of a table with the same name", ErrorCodes::LOGICAL_ERROR);
                    }
                    for (const String & disk_name : all_diff_disk_names)
                    {
                        auto disk = new_storage_policy->getDiskByName(disk_name);
                        disk->createDirectories(relative_data_path);
                        disk->createDirectories(fs::path(relative_data_path) / MergeTreeData::DETACHED_DIR_NAME);
                    }
                    
                    has_storage_policy_changed = true;
                }
            }
        }
        MergeTreeSettings copy = *getSettings();
        copy.applyChanges(new_changes);
        copy.sanityCheck(getContext()->getSettingsRef());
        storage_settings.set(std::make_unique<const MergeTreeSettings>(copy));
        StorageInMemoryMetadata new_metadata = getInMemoryMetadata();
        new_metadata.setSettingsChanges(new_settings);
        setInMemoryMetadata(new_metadata);
        if (has_storage_policy_changed)
            startBackgroundMovesIfNeeded();
    }
}
void MergeTreeData::PartsTemporaryRename::addPart(const String & old_name, const String & new_name)