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. - 

dst_parts.emplace_back(dest_table_storage->cloneAndLoadDataPartOnSameDisk(src_part, TMP_PREFIX, dst_part_info, dest_metadata_snapshot));
    }
    
    if (dst_parts.empty())
        return;
    
    try
    {
        {
            Transaction transaction(*dest_table_storage);
            auto src_data_parts_lock = lockParts();
            auto dest_data_parts_lock = dest_table_storage->lockParts();
            std::mutex mutex;
            DataPartsLock lock(mutex);
            for (MutableDataPartPtr & part : dst_parts)
                dest_table_storage->renameTempPartAndReplace(part, &dest_table_storage->increment, &transaction, lock);
            removePartsFromWorkingSet(src_parts, true, lock);
            transaction.commit(&lock);
        }
        clearOldMutations(true);
        clearOldPartsFromFilesystem();
        PartLog::addNewParts(getContext(), dst_parts, watch.elapsed());
    }
    catch (...)
    {
        PartLog::addNewParts(getContext(), dst_parts, watch.elapsed(), ExecutionStatus::fromCurrentException());
        throw;
    }
}
ActionLock StorageMergeTree::getActionLock(StorageActionBlockType action_type)