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. - 

MergeTreeData::getAllDataPartsVector(MergeTreeData::DataPartStateVector * out_states, bool require_projection_parts) const
{
    DataPartsVector res;
    if (require_projection_parts)
    {
        auto lock = lockParts();
        for (const auto & part : data_parts_by_info)
        {
            for (const auto & [p_name, projection_part] : part->getProjectionParts())
                res.push_back(projection_part);
        }
        if (out_states != nullptr)
        {
            out_states->resize(res.size());
            for (size_t i = 0; i < res.size(); ++i)
                (*out_states)[i] = res[i]->getParentPart()->getState();
        }
    }
    else
    {
        auto lock = lockParts();
        res.assign(data_parts_by_info.begin(), data_parts_by_info.end());
        if (out_states != nullptr)
        {
            out_states->resize(res.size());
            for (size_t i = 0; i < res.size(); ++i)
                (*out_states)[i] = res[i]->getState();
        }
    }
    return res;