**Describe the bug**

Since we upgraded to 20.10.3.30, one of our clickhouse cluster takes much more time to start. (Up to 50 minutes, and before it took ~3 minutes)
The log file is filled with
>  <Debug> DiskLocal: Reserving 1.00 MiB on disk `default`, having unreserved 16.90 TiB

Additionally, the memory consumption increases, linearly, a lot (up to arround 230 G for us) and then drop back to normal values once clickhouse finished starting up. During that loading phase, a single CPU core is used at 100%.
If that memory is not available for the machine, clickhouse gets OOMed by the kernel.


**How to reproduce**
* Issue appeared after we upgraded to ClickHouse server version 20.10.3.30

* How could I find wich of our tables may be responsible for that ?


**Expected behavior**
A fast startup :)

return true;
        if (lower != block_numbers_on_disk.begin())
        {
            --lower;
            if (min_block >= lower->first && max_block <= lower->second)
                return true;
        }
        return false;
    };
    size_t cleared_count = 0;
    auto disks = getStoragePolicy()->getDisks();
    for (auto disk_it = disks.rbegin(); disk_it != disks.rend(); ++disk_it)
    {
        auto disk_ptr = *disk_it;
        if (disk_ptr->isBroken())
            continue;
        for (auto it = disk_ptr->iterateDirectory(relative_data_path); it->isValid(); it->next())
        {
            auto min_max_block_number = MergeTreeWriteAheadLog::tryParseMinMaxBlockNumber(it->name());
            if (min_max_block_number && is_range_on_disk(min_max_block_number->first, min_max_block_number->second))
            {
                LOG_DEBUG(log, "Removing from filesystem the outdated WAL file {}", it->name());
                disk_ptr->removeFile(relative_data_path + it->name());
                ++cleared_count;
            }
        }
    }
    return cleared_count;
}
size_t MergeTreeData::clearEmptyParts()