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. - 

*/
    bool executeLogEntry(LogEntry & entry);
    
    
    MutableDataPartPtr attachPartHelperFoundValidPart(const LogEntry& entry) const;
    void executeDropRange(const LogEntry & entry);
    
    bool tryExecuteMerge(const LogEntry & entry);
    
    
    
    bool executeMetadataAlter(const LogEntry & entry);
    
    
    bool tryExecutePartMutation(const LogEntry & entry);
    
    
    
    
    bool executeFetch(LogEntry & entry);
    bool executeReplaceRange(const LogEntry & entry);
    void executeClonePartFromShard(const LogEntry & entry);
    /** Updates the queue.
      */
    void queueUpdatingTask();
    void mutationsUpdatingTask();
    /** Clone data from another replica.
      * If replica can not be cloned throw Exception.
      */
    void cloneReplica(const String & source_replica, Coordination::Stat source_is_lost_stat, zkutil::ZooKeeperPtr & zookeeper);