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. - 

* Returns false if part is already fetching right now.
      */
    bool fetchPart(
        const String & part_name,
        const StorageMetadataPtr & metadata_snapshot,
        const String & replica_path,
        bool to_detached,
        size_t quorum,
        zkutil::ZooKeeper::Ptr zookeeper_ = nullptr);
    /** Download the specified part from the specified replica.
      * Used for replace local part on the same s3-shared part in hybrid storage.
      * Returns false if part is already fetching right now.
      */
    bool fetchExistsPart(
        const String & part_name,
        const StorageMetadataPtr & metadata_snapshot,
        const String & replica_path,
        DiskPtr replaced_disk,
        String replaced_part_path);
    
    std::unordered_set<String> currently_fetching_parts;
    std::mutex currently_fetching_parts_mutex;
    
    void updateQuorum(const String & part_name, bool is_parallel);
    
    void cleanLastPartNode(const String & partition_id);
    
    bool partIsLastQuorumPart(const MergeTreePartInfo & part_info) const;
    
    bool partIsInsertingWithParallelQuorum(const MergeTreePartInfo & part_info) const;