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 String & name, const VolumePtr & volume, const String & relative_path, const IMergeTreeDataPart * parent_part) const
{
    return createPart(name, MergeTreePartInfo::fromPartName(name, format_version), volume, relative_path, parent_part);
}
MergeTreeData::MutableDataPartPtr MergeTreeData::createPart(
    const String & name, const MergeTreePartInfo & part_info,
    const VolumePtr & volume, const String & relative_path, const IMergeTreeDataPart * parent_part) const
{
    MergeTreeDataPartType type;
    auto full_path = fs::path(relative_data_path) / (parent_part ? parent_part->relative_path : "") / relative_path / "";
    auto mrk_ext = MergeTreeIndexGranularityInfo::getMarksExtensionFromFilesystem(volume->getDisk(), full_path);
    if (mrk_ext)
        type = getPartTypeFromMarkExtension(*mrk_ext);
    else
    {
        
        type = choosePartTypeOnDisk(0, 0);
    }
    return createPart(name, type, part_info, volume, relative_path, parent_part);
}
void MergeTreeData::changeSettings(