**Describe the solution you'd like**

We already can run clickhouse-server without configured ZooKeeper (the tables will start in read-only mode)
and with configured ZooKeeper, if it starts to be inaccessible at runtime (the tables will be read-only until the session is re-established).

Let's also make it possible to start up when ZooKeeper is unavailable.

**Additional context**

If that will be very hard to implement, we can consider other options:
- lazy attaching of the tables after sever start-up;
- a maintenance mode when the tables are not attached at start-up and require manual ATTACH queries.
{
    return config.getString("default_replica_path", "/clickhouse/tables/{uuid}/{shard}");
}
String StorageReplicatedMergeTree::getDefaultReplicaName(const Poco::Util::AbstractConfiguration & config)
{
    return config.getString("default_replica_name", "{replica}");
}
bool StorageReplicatedMergeTree::checkFixedGranularityInZookeeper()
{
    auto zookeeper = getZooKeeper();
    String metadata_str = zookeeper->get(zookeeper_path + "/metadata");
    auto metadata_from_zk = ReplicatedMergeTreeTableMetadata::parse(metadata_str);
    return metadata_from_zk.index_granularity_bytes == 0;
}
void StorageReplicatedMergeTree::waitMutationToFinishOnReplicas(