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. - 

other_replicas_fixed_granularity = checkFixedGranualrityInZookeeper();
            ReplicatedMergeTreeTableMetadata current_metadata(*this, metadata_snapshot);
            current_zookeeper->createOrUpdate(replica_path + "/metadata", current_metadata.toString(), zkutil::CreateMode::Persistent);
        }
        checkTableStructure(replica_path, metadata_snapshot);
        checkParts(skip_sanity_checks);
        if (current_zookeeper->exists(replica_path + "/metadata_version"))
        {
            metadata_version = parse<int>(current_zookeeper->get(replica_path + "/metadata_version"));
        }
        else
        {
            
            
            
            
            
            Coordination::Stat metadata_stat;
            current_zookeeper->get(zookeeper_path + "/metadata", &metadata_stat);
            metadata_version = metadata_stat.version;
        }
        
        
        clearOldTemporaryDirectories(0);
        clearOldWriteAheadLogs();
    }
    createNewZooKeeperNodes();
    syncPinnedPartUUIDs();
}
bool StorageReplicatedMergeTree::checkFixedGranualrityInZookeeper()