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. - 

DataPartsVector renameTempPartAndReplace(
        MutableDataPartPtr & part, SimpleIncrement * increment = nullptr, Transaction * out_transaction = nullptr, MergeTreeDeduplicationLog * deduplication_log = nullptr);
    
    bool renameTempPartAndReplace(
            MutableDataPartPtr & part, SimpleIncrement * increment, Transaction * out_transaction, DataPartsLock & lock,
            DataPartsVector * out_covered_parts = nullptr, MergeTreeDeduplicationLog * deduplication_log = nullptr);
    
    
    
    void removePartsFromWorkingSetImmediatelyAndSetTemporaryState(const DataPartsVector & remove);
    
    
    
    
    void removePartsFromWorkingSet(const DataPartsVector & remove, bool clear_without_timeout, DataPartsLock * acquired_lock = nullptr);
    void removePartsFromWorkingSet(const DataPartsVector & remove, bool clear_without_timeout, DataPartsLock & acquired_lock);
    
    
    
    DataPartsVector removePartsInRangeFromWorkingSet(const MergeTreePartInfo & drop_range, bool clear_without_timeout,
                                                     DataPartsLock & lock);
    
    
    
    void forgetPartAndMoveToDetached(const DataPartPtr & part, const String & prefix = "", bool restore_covered = false);
    
    void tryRemovePartImmediately(DataPartPtr && part);