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();
    zookeeper->createAncestors(zookeeper_path);
    for (size_t i = 0; i < 1000; ++i)
    {
        
        if (zookeeper->exists(zookeeper_path + "/replicas"))
        {
            LOG_DEBUG(log, "This table {} is already created, will add new replica", zookeeper_path);
            return false;
        }