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. - 

LOG_ERROR(log, "No ZooKeeper: table will be in readonly mode.");
        is_readonly = true;
        return;
    }
    if (attach && !current_zookeeper->exists(zookeeper_path + "/metadata"))
    {
        LOG_WARNING(log, "No metadata in ZooKeeper: table will be in readonly mode.");
        is_readonly = true;
        has_metadata_in_zookeeper = false;
        return;
    }
    auto metadata_snapshot = getInMemoryMetadataPtr();
    if (!attach)
    {
        if (!getDataParts().empty())
            throw Exception("Data directory for table already containing data parts"
                " - probably it was unclean DROP table or manual intervention."
                " You must either clear directory by hand or use ATTACH TABLE"
                " instead of CREATE TABLE if you need to use that parts.", ErrorCodes::INCORRECT_DATA);
        try
        {
            bool is_first_replica = createTableIfNotExists(metadata_snapshot);
            try
            {
                
                
                
                
                other_replicas_fixed_granularity = checkFixedGranualrityInZookeeper();