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 can_become_leader;
        bool is_readonly;
        bool is_session_expired;
        ReplicatedMergeTreeQueue::Status queue;
        UInt32 parts_to_check;
        String zookeeper_path;
        String replica_name;
        String replica_path;
        Int32 columns_version;
        UInt64 log_max_index;
        UInt64 log_pointer;
        UInt64 absolute_delay;
        UInt8 total_replicas;
        UInt8 active_replicas;
        
        String zookeeper_exception;
    };
    
    void getStatus(Status & res, bool with_zk_fields = true);
    using LogEntriesData = std::vector<ReplicatedMergeTreeLogEntryData>;
    void getQueue(LogEntriesData & res, String & replica_name);
    std::vector<PartMovesBetweenShardsOrchestrator::Entry> getPartMovesBetweenShardsEntries();
    
    time_t getAbsoluteDelay() const;
    
    
    
    void getReplicaDelays(time_t & out_absolute_delay, time_t & out_relative_delay);
    
    void enqueuePartForCheck(const String & part_name, time_t delay_to_check_seconds = 0)