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 (force)
    {
        
        auto merge_blocker = stopMergesAndWait();
        auto part = getPartIfExists(part_name, {MergeTreeDataPartState::Committed});
        if (!part)
            throw Exception("Part " + part_name + " not found, won't try to drop it.", ErrorCodes::NO_SUCH_DATA_PART);
        removePartsFromWorkingSet({part}, true);
        return part;
    }
    else
    {
        
        std::unique_lock lock(currently_processing_in_background_mutex);
        auto part = getPartIfExists(part_name, {MergeTreeDataPartState::Committed});
        
        if (!part)
            return nullptr;
        
        
        if (currently_merging_mutating_parts.count(part))
            return nullptr;
        removePartsFromWorkingSet({part}, true);
        return part;
    }
}
void StorageMergeTree::dropPartition(const ASTPtr & partition, bool detach, bool drop_part, ContextPtr local_context, bool throw_if_noop)