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. - 

for (const auto & [name, disk] : name_to_disk)
        {
            const String containing_part = active_parts.getContainingPart(name);
            if (!containing_part.empty() && containing_part != name)
                /
                disk->moveDirectory(fs::path(relative_data_path) / source_dir / name,
                    fs::path(relative_data_path) / source_dir / ("inactive_" + name));
            else
                renamed_parts.addPart(name, "attaching_" + name);
        }
    }
    /
    renamed_parts.tryRenameAll();
    /
    LOG_DEBUG(log, "Checking parts");
    MutableDataPartsVector loaded_parts;
    loaded_parts.reserve(renamed_parts.old_and_new_names.size());
    for (const auto & [old_name, new_name] : renamed_parts.old_and_new_names)
    {
        LOG_DEBUG(log, "Checking part {}", new_name);
        auto single_disk_volume = std::make_shared<SingleDiskVolume>("volume_" + old_name, name_to_disk[old_name]);
        MutableDataPartPtr part = createPart(old_name, single_disk_volume, source_dir + new_name);
        loadPartAndFixMetadataImpl(part);
        loaded_parts.push_back(part);
    }
    return loaded_parts;
}
namespace
{
inline ReservationPtr checkAndReturnReservation(UInt64 expected_size, ReservationPtr reservation)