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. - 

{
    fs::directory_iterator dir_end;
    for (const std::string & path : getDataPaths())
        for (fs::directory_iterator dir_it{fs::path(path) / "detached/"}; dir_it != dir_end; ++dir_it)
            if (dir_it->path().filename().string() == part_name)
                return true;
    return false;
}
bool StorageReplicatedMergeTree::checkIfDetachedPartitionExists(const String & partition_name)
{
    fs::directory_iterator dir_end;
    for (const std::string & path : getDataPaths())
    {
        for (fs::directory_iterator dir_it{fs::path(path) / "detached/"}; dir_it != dir_end; ++dir_it)
        {
            MergeTreePartInfo part_info;
            if (MergeTreePartInfo::tryParsePartName(dir_it->path().filename(), &part_info, format_version) && part_info.partition_id == partition_name)
                return true;
        }
    }
    return false;
}
}