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. - 

Strings replicas = zookeeper->getChildren(fs::path(zookeeper_path) / "replicas");
    std::vector<zkutil::ZooKeeper::FutureGet> futures;
    for (const String & source_replica_name : replicas)
    {
        
        if (source_replica_name == replica_name)
            continue;
        String source_replica_path = fs::path(zookeeper_path) / "replicas" / source_replica_name;
        
        
        futures.emplace_back(zookeeper->asyncTryGet(fs::path(source_replica_path) / "is_lost"));
        futures.emplace_back(zookeeper->asyncTryGet(fs::path(source_replica_path) / "log_pointer"));
        futures.emplace_back(zookeeper->asyncTryGet(fs::path(source_replica_path) / "queue"));
    }
    
    for (auto & future : futures)
        future.wait();
    Strings log_entries = zookeeper->getChildren(fs::path(zookeeper_path) / "log");
    size_t max_log_entry = 0;
    if (!log_entries.empty())
    {
        String last_entry = *std::max_element(log_entries.begin(), log_entries.end());
        max_log_entry = parse<UInt64>(last_entry.substr(strlen("log-")));
    }
    
    ++max_log_entry;
    size_t min_replication_lag = std::numeric_limits<size_t>::max();