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. - 

{
    auto zookeeper = getZooKeeper();
    LOG_DEBUG(log, "Creating replica {}", replica_path);
    Coordination::Error code;
    do
    {
        Coordination::Stat replicas_stat;
        String replicas_value;
        if (!zookeeper->tryGet(zookeeper_path + "/replicas", replicas_value, &replicas_stat))
            throw Exception(fmt::format("Cannot create a replica of the table {}, because the last replica of the table was dropped right now",
                zookeeper_path), ErrorCodes::ALL_REPLICAS_LOST);
        
        
        String is_lost_value = replicas_stat.numChildren ? "1" : "0";
        Coordination::Requests ops;
        ops.emplace_back(zkutil::makeCreateRequest(replica_path, "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/host", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/log_pointer", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/queue", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/parts", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/flags", "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(replica_path + "/is_lost", is_lost_value,