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. - 

String source_replica;
    Coordination::Stat source_is_lost_stat;
    size_t future_num = 0;
    for (const String & source_replica_name : replicas)
    {
        if (source_replica_name == replica_name)
            continue;
        auto get_is_lost     = futures[future_num++].get();
        auto get_log_pointer = futures[future_num++].get();
        auto get_queue       = futures[future_num++].get();
        if (get_is_lost.error != Coordination::Error::ZOK)
        {
            LOG_INFO(log, "Not cloning {}, cannot get '/is_lost': {}", source_replica_name, Coordination::errorMessage(get_is_lost.error));
            continue;
        }
        else if (get_is_lost.data != "0")
        {
            LOG_INFO(log, "Not cloning {}, it's lost", source_replica_name);
            continue;
        }
        if (get_log_pointer.error != Coordination::Error::ZOK)
        {
            LOG_INFO(log, "Not cloning {}, cannot get '/log_pointer': {}", source_replica_name, Coordination::errorMessage(get_log_pointer.error));
            continue;
        }
        if (get_queue.error != Coordination::Error::ZOK)
        {
            LOG_INFO(log, "Not cloning {}, cannot get '/queue': {}", source_replica_name, Coordination::errorMessage(get_queue.error));
            continue;
        }