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. - 

checkTableStructure(zookeeper_path, metadata_snapshot);
                Coordination::Stat metadata_stat;
                current_zookeeper->get(zookeeper_path + "/metadata", &metadata_stat);
                metadata_version = metadata_stat.version;
            }
            catch (Coordination::Exception & e)
            {
                if (!is_first_replica && e.code == Coordination::Error::ZNONODE)
                    throw Exception("Table " + zookeeper_path + " was suddenly removed.", ErrorCodes::ALL_REPLICAS_LOST);
                else
                    throw;
            }
            if (!is_first_replica)
                createReplica(metadata_snapshot);
        }
        catch (...)
        {
            
            dropIfEmpty();
            throw;
        }
    }
    else
    {
        
        String replica_metadata;
        bool replica_metadata_exists = current_zookeeper->tryGet(replica_path + "/metadata", replica_metadata);
        if (!replica_metadata_exists || replica_metadata.empty())
        {