**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.
{
public:
    enum RenamingRestrictions
    {
        ALLOW_ANY,
        ALLOW_PRESERVING_UUID,
        DO_NOT_ALLOW,
    };
    /** If not 'attach', either creates a new table in ZK, or adds a replica to an existing table.
      */
    StorageReplicatedMergeTree(
        const String & zookeeper_path_,
        const String & replica_name_,
        bool attach,
        const StorageID & table_id_,
        const String & relative_data_path_,
        const StorageInMemoryMetadata & metadata_,
        ContextMutablePtr context_,
        const String & date_column_name,
        const MergingParams & merging_params_,
        std::unique_ptr<MergeTreeSettings> settings_,
        bool has_force_restore_data_flag,
        RenamingRestrictions renaming_restrictions_);
    void startup() override;
    void shutdown() override;
    void flush() override;
    ~StorageReplicatedMergeTree() override;
    static String getDefaultZooKeeperPath(const Poco::Util::AbstractConfiguration & config);
    static String getDefaultReplicaName(const Poco::Util::AbstractConfiguration & config);
    std::string getName() const override { return "Replicated" + merging_params.getModeName() + "MergeTree"; }