**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 :)

{
    if (parts_mover.moves_blocker.isCancelled())
        return false;
    auto moving_tagger = checkPartsForMove(parts, space);
    if (moving_tagger->parts_to_move.empty())
        return false;
    return moveParts(moving_tagger);
}
MergeTreeData::CurrentlyMovingPartsTaggerPtr MergeTreeData::selectPartsForMove()
{
    MergeTreeMovingParts parts_to_move;
    auto can_move = [this](const DataPartPtr & part, String * reason) -> bool
    {
        if (partIsAssignedToBackgroundOperation(part))
        {
            *reason = "part already assigned to background operation.";
            return false;
        }
        if (currently_moving_parts.contains(part))
        {
            *reason = "part is already moving.";
            return false;
        }
        return true;
    };
    std::lock_guard moving_lock(moving_parts_mutex);
    parts_mover.selectPartsForMove(parts_to_move, can_move, moving_lock);
    return std::make_shared<CurrentlyMovingPartsTagger>(std::move(parts_to_move), *this);
}
MergeTreeData::CurrentlyMovingPartsTaggerPtr MergeTreeData::checkPartsForMove(const DataPartsVector & parts, SpacePtr space)