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 & metadata_snapshot,
    bool attach_part,
    ContextPtr query_context)
{
    assertNotReadonly();
    PartitionCommandsResultInfo results;
    PartsTemporaryRename renamed_parts(*this, "detached/");
    MutableDataPartsVector loaded_parts = tryLoadPartsToAttach(partition, attach_part, query_context, renamed_parts);
    
    ReplicatedMergeTreeBlockOutputStream output(*this, metadata_snapshot, 0, 0, 0, false, false, query_context,
        true);
    for (size_t i = 0; i < loaded_parts.size(); ++i)
    {
        const String old_name = loaded_parts[i]->name;
        output.writeExistingPart(loaded_parts[i]);
        renamed_parts.old_and_new_names[i].first.clear();
        LOG_DEBUG(log, "Attached part {} as {}", old_name, loaded_parts[i]->name);
        results.push_back(PartitionCommandResultInfo{
            .partition_id = loaded_parts[i]->info.partition_id,
            .part_name = loaded_parts[i]->name,
            .old_part_name = old_name,
        });
    }
    return results;
}
void StorageReplicatedMergeTree::checkTableCanBeDropped() const