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. - 

std::optional<MergeTreeMutationStatus> getIncompleteMutationsStatus(Int64 mutation_version, std::set<String> * mutation_ids = nullptr) const;
    void startBackgroundMovesIfNeeded() override;
    friend class MergeTreeProjectionBlockOutputStream;
    friend class MergeTreeBlockOutputStream;
    friend class MergeTreeData;
protected:
    /** Attach the table with the appropriate name, along the appropriate path (with / at the end),
      *  (correctness of names and paths are not checked)
      *  consisting of the specified columns.
      *
      * See MergeTreeData constructor for comments on parameters.
      */
    StorageMergeTree(
        const StorageID & table_id_,
        const String & relative_data_path_,
        const StorageInMemoryMetadata & metadata,
        bool attach,
        ContextMutablePtr context_,
        const String & date_column_name,
        const MergingParams & merging_params_,
        std::unique_ptr<MergeTreeSettings> settings_,
        bool has_force_restore_data_flag);
    MutationCommands getFirstAlterMutationCommandsForPart(const DataPartPtr & part) const override;
};
}