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 (disk->getLastModified(directory_path).epochTime() >= threshold)
        return false;
    for (auto it = disk->iterateDirectory(directory_path); it->isValid(); it->next())
        if (disk->getLastModified(it->path()).epochTime() >= threshold)
            return false;
    return true;
}
void MergeTreeData::clearOldTemporaryDirectories(ssize_t custom_directories_lifetime_seconds)
{
    
    std::unique_lock lock(clear_old_temporary_directories_mutex, std::defer_lock);
    if (!lock.try_lock())
        return;
    const auto settings = getSettings();
    time_t current_time = time(nullptr);
    ssize_t deadline = (custom_directories_lifetime_seconds >= 0)
        ? current_time - custom_directories_lifetime_seconds