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 SelectQueryInfo & query_info, ContextPtr local_context, const DataPartsVector & parts) const
{
    if (parts.empty())
        return 0u;
    auto metadata_snapshot = getInMemoryMetadataPtr();
    ASTPtr expression_ast;
    Block virtual_columns_block = getBlockWithVirtualPartColumns(parts, true );
    
    bool valid = VirtualColumnUtils::prepareFilterBlockWithQuery(query_info.query, local_context, virtual_columns_block, expression_ast);
    PartitionPruner partition_pruner(metadata_snapshot, query_info, local_context, true );
    if (partition_pruner.isUseless() && !valid)
        return {};
    std::unordered_set<String> part_values;
    if (valid && expression_ast)
    {
        virtual_columns_block = getBlockWithVirtualPartColumns(parts, false );
        VirtualColumnUtils::filterBlockWithQuery(query_info.query, virtual_columns_block, local_context, expression_ast);
        part_values = VirtualColumnUtils::extractSingleValueFromBlock<String>(virtual_columns_block, "_part");
        if (part_values.empty())
            return 0;
    }
    
    size_t res = 0;