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 ASTPtr & partition, const StorageMetadataPtr & ,
    bool attach_part, ContextPtr local_context)
{
    PartitionCommandsResultInfo results;
    PartsTemporaryRename renamed_parts(*this, "detached/");
    MutableDataPartsVector loaded_parts = tryLoadPartsToAttach(partition, attach_part, local_context, renamed_parts);
    for (size_t i = 0; i < loaded_parts.size(); ++i)
    {
        LOG_INFO(log, "Attaching part {} from {}", loaded_parts[i]->name, renamed_parts.old_and_new_names[i].second);
        String old_name = renamed_parts.old_and_new_names[i].first;
        renameTempPartAndAdd(loaded_parts[i], &increment);
        renamed_parts.old_and_new_names[i].first.clear();
        results.push_back(PartitionCommandResultInfo{
            .partition_id = loaded_parts[i]->info.partition_id,
            .part_name = loaded_parts[i]->name,
            .old_part_name = old_name,
        });
        LOG_INFO(log, "Finished attaching part");
    }
    
    local_context->dropCaches();
    return results;
}
void StorageMergeTree::replacePartitionFrom(const StoragePtr & source_table, const ASTPtr & partition, bool replace, ContextPtr local_context)