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. - 

{
        const auto & src_relative_data_path = src_part_in_memory->storage.relative_data_path;
        auto flushed_part_path = src_part_in_memory->getRelativePathForPrefix(tmp_part_prefix);
        src_part_in_memory->flushToDisk(src_relative_data_path, flushed_part_path, metadata_snapshot);
        src_part_path = fs::path(src_relative_data_path) / flushed_part_path / "";
    }
    LOG_DEBUG(log, "Cloning part {} to {}", fullPath(disk, src_part_path), fullPath(disk, dst_part_path));
    localBackup(disk, src_part_path, dst_part_path);
    disk->removeFileIfExists(fs::path(dst_part_path) / IMergeTreeDataPart::DELETE_ON_DESTROY_MARKER_FILE_NAME);
    auto single_disk_volume = std::make_shared<SingleDiskVolume>(disk->getName(), disk, 0);
    auto dst_data_part = createPart(dst_part_name, dst_part_info, single_disk_volume, tmp_dst_part_name);
    dst_data_part->is_temp = true;
    dst_data_part->loadColumnsChecksumsIndexes(require_part_metadata, true);
    dst_data_part->modification_time = disk->getLastModified(dst_part_path).epochTime();
    return dst_data_part;
}
String MergeTreeData::getFullPathOnDisk(const DiskPtr & disk) const
{
    return disk->getPath() + relative_data_path;
}
DiskPtr MergeTreeData::getDiskForPart(const String & part_name, const String & additional_path) const
{
    const auto disks = getStoragePolicy()->getDisks();
    for (const DiskPtr & disk : disks)
        for (auto it = disk->iterateDirectory(relative_data_path + additional_path); it->isValid(); it->next())
            if (it->name() == part_name)
                return disk;
    return nullptr;
}
std::optional<String> MergeTreeData::getFullRelativePathForPart(const String & part_name, const String & additional_path) const