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. - 

DataParts currently_moving_parts;
    
    mutable std::mutex moving_parts_mutex;
    PinnedPartUUIDsPtr getPinnedPartUUIDs() const;
    
    virtual std::optional<JobAndPool> getDataProcessingJob() = 0;
    
    std::optional<JobAndPool> getDataMovingJob();
    bool areBackgroundMovesNeeded() const;
    
    
    virtual void lockSharedData(const IMergeTreeDataPart &) const {}
    
    
    virtual bool unlockSharedData(const IMergeTreeDataPart &) const { return true; }
    
    
    virtual bool tryToFetchIfShared(const IMergeTreeDataPart &, const DiskPtr &, const String &) { return false; }
    
    
    
    DataParts currently_submerging_big_parts;
    std::map<String, EmergingPartInfo> currently_emerging_big_parts;
    
    mutable std::mutex currently_submerging_emerging_mutex;