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 (!res.empty())
        LOG_TRACE(log, "Found {} old parts to remove.", res.size());
    return res;
}
void MergeTreeData::rollbackDeletingParts(const MergeTreeData::DataPartsVector & parts)
{
    auto lock = lockParts();
    for (const auto & part : parts)
    {
        
        part->assertState({DataPartState::Deleting});
        modifyPartState(part, DataPartState::Outdated);
    }
}
void MergeTreeData::removePartsFinally(const MergeTreeData::DataPartsVector & parts)
{
    {
        auto lock = lockParts();
        
        for (const auto & part : parts)
        {
            auto it = data_parts_by_info.find(part->info);
            if (it == data_parts_by_info.end())
                throw Exception("Deleting data part " + part->name + " doesn't exist", ErrorCodes::LOGICAL_ERROR);
            (*it)->assertState({DataPartState::Deleting});
            data_parts_indexes.erase(it);
        }
    }