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. - 

{
    PartsTemporaryRename renamed_parts(*this, "detached/");
    if (part)
    {
        String part_name = partition->as<ASTLiteral &>().value.safeGet<String>();
        validateDetachedPartName(part_name);
        renamed_parts.addPart(part_name, "deleting_" + part_name);
    }
    else
    {
        String partition_id = getPartitionIDFromQuery(partition, local_context);
        DetachedPartsInfo detached_parts = getDetachedParts();
        for (const auto & part_info : detached_parts)
            if (part_info.valid_name && part_info.partition_id == partition_id
                && part_info.prefix != "attaching" && part_info.prefix != "deleting")
                renamed_parts.addPart(part_info.dir_name, "deleting_" + part_info.dir_name);
    }
    LOG_DEBUG(log, "Will drop {} detached parts.", renamed_parts.old_and_new_names.size());
    renamed_parts.tryRenameAll();
    for (auto & [old_name, new_name] : renamed_parts.old_and_new_names)
    {
        const auto & [path, disk] = renamed_parts.old_part_name_to_path_and_disk[old_name];
        disk->removeRecursive(fs::path(path) / "detached" / new_name / "");
        LOG_DEBUG(log, "Dropped detached part {}", old_name);
        old_name.clear();
    }
}
MergeTreeData::MutableDataPartsVector MergeTreeData::tryLoadPartsToAttach(const ASTPtr & partition, bool attach_part,