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 & node, const StorageMetadataPtr & metadata_snapshot) const
{
    const String column_name = node->getColumnName();
    for (const auto & name : metadata_snapshot->getPrimaryKeyColumns())
        if (column_name == name)
            return true;
    for (const auto & name : getMinMaxColumnsNames(metadata_snapshot->getPartitionKey()))
        if (column_name == name)
            return true;
    if (const auto * func = node->as<ASTFunction>())
        if (func->arguments->children.size() == 1)
            return isPrimaryOrMinMaxKeyColumnPossiblyWrappedInFunctions(func->arguments->children.front(), metadata_snapshot);
    return false;
}
bool MergeTreeData::mayBenefitFromIndexForIn(