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. - 

const ASTPtr & left_in_operand, ContextPtr, const StorageMetadataPtr & metadata_snapshot) const
{
    /
    /
    /
    const auto * left_in_operand_tuple = left_in_operand->as<ASTFunction>();
    const auto & index_wrapper_factory = MergeTreeIndexFactory::instance();
    if (left_in_operand_tuple && left_in_operand_tuple->name == "tuple")
    {
        for (const auto & item : left_in_operand_tuple->arguments->children)
        {
            if (isPrimaryOrMinMaxKeyColumnPossiblyWrappedInFunctions(item, metadata_snapshot))
                return true;
            for (const auto & index : metadata_snapshot->getSecondaryIndices())
                if (index_wrapper_factory.get(index)->mayBenefitFromIndexForIn(item))
                    return true;
            if (metadata_snapshot->selected_projection
                && metadata_snapshot->selected_projection->isPrimaryKeyColumnPossiblyWrappedInFunctions(item))
                return true;