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. - 

nullptr);
        };
        try
        {
            cloned_part = parts_mover.clonePart(moving_part);
            parts_mover.swapClonedPart(cloned_part);
            write_part_log({});
        }
        catch (...)
        {
            write_part_log(ExecutionStatus::fromCurrentException());
            if (cloned_part)
                cloned_part->remove();
            throw;
        }
    }
    return true;
}
bool MergeTreeData::partsContainSameProjections(const DataPartPtr & left, const DataPartPtr & right)
{
    if (left->getProjectionParts().size() != right->getProjectionParts().size())
        return false;
    for (const auto & [name, _] : left->getProjectionParts())
    {
        if (!right->hasProjection(name))
            return false;
    }
    return true;
}
bool MergeTreeData::canUsePolymorphicParts(const MergeTreeSettings & settings, String * out_reason) const