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. - 

{
            return std::forward_as_tuple(static_cast<UInt8>(lhs.state), lhs.partition_id)
                   < std::forward_as_tuple(static_cast<UInt8>(rhs.state), rhs.info.partition_id);
        }
    };
    using DataParts = std::set<DataPartPtr, LessDataPart>;
    using DataPartsVector = std::vector<DataPartPtr>;
    using DataPartsLock = std::unique_lock<std::mutex>;
    DataPartsLock lockParts() const { return DataPartsLock(data_parts_mutex); }
    MergeTreeDataPartType choosePartType(size_t bytes_uncompressed, size_t rows_count) const;
    MergeTreeDataPartType choosePartTypeOnDisk(size_t bytes_uncompressed, size_t rows_count) const;
    
    MutableDataPartPtr createPart(const String & name,
        MergeTreeDataPartType type, const MergeTreePartInfo & part_info,
        const VolumePtr & volume, const String & relative_path, const IMergeTreeDataPart * parent_part = nullptr) const;
    
    
    MutableDataPartPtr createPart(const String & name,
        const VolumePtr & volume, const String & relative_path, const IMergeTreeDataPart * parent_part = nullptr) const;
    MutableDataPartPtr createPart(const String & name, const MergeTreePartInfo & part_info,
        const VolumePtr & volume, const String & relative_path, const IMergeTreeDataPart * parent_part = nullptr) const;
    
    
    
    
    
    class Transaction : private boost::noncopyable