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. - 

{
    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.count(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)