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. - 

size_t index_,
            const String & src_part_name_,
            const String & new_part_name_,
            const String & checksum_hex_,
            MergeTreeDataFormatVersion format_version)
            : index(index_)
            , src_part_name(src_part_name_)
            , src_part_info(MergeTreePartInfo::fromPartName(src_part_name_, format_version))
            , new_part_name(new_part_name_)
            , new_part_info(MergeTreePartInfo::fromPartName(new_part_name_, format_version))
            , checksum_hex(checksum_hex_)
        {
        }
        size_t index; 
        String src_part_name;
        MergeTreePartInfo src_part_info;
        String new_part_name;
        MergeTreePartInfo new_part_info;
        String checksum_hex;
        
        MutableDataPartPtr res_part;