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::DataPartsVector parts_to_remove;
        auto metadata_snapshot = getInMemoryMetadataPtr();
        if (drop_part)
        {
            auto part = outdatePart(partition->as<ASTLiteral &>().value.safeGet<String>(), throw_if_noop);
            
            if (!part)
                return;
            parts_to_remove.push_back(part);
        }
        else
        {