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. - 

void swapActivePart(MergeTreeData::DataPartPtr part_copy);
    
    DataPartsVector getDataPartsVectorInPartition(DataPartState state, const String & partition_id);
    
    DataPartPtr getPartIfExists(const String & part_name, const DataPartStates & valid_states);
    DataPartPtr getPartIfExists(const MergeTreePartInfo & part_info, const DataPartStates & valid_states);
    
    size_t getTotalActiveSizeInBytes() const;
    size_t getTotalActiveSizeInRows() const;
    size_t getPartsCount() const;
    size_t getMaxPartsCountForPartitionWithState(DataPartState state) const;
    size_t getMaxPartsCountForPartition() const;
    size_t getMaxInactivePartsCountForPartition() const;
    
    
    std::optional<Int64> getMinPartDataVersion() const;
    
    
    void delayInsertOrThrowIfNeeded(Poco::Event * until = nullptr) const;
    
    
    
    
    
    
    
    bool renameTempPartAndAdd(MutableDataPartPtr & part, SimpleIncrement * increment = nullptr, Transaction * out_transaction = nullptr, MergeTreeDeduplicationLog * deduplication_log = nullptr);