**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.
mutable std::mutex current_zookeeper_mutex;    
    zkutil::ZooKeeperPtr tryGetZooKeeper() const;
    zkutil::ZooKeeperPtr getZooKeeper() const;
    zkutil::ZooKeeperPtr getZooKeeperAndAssertNotReadonly() const;
    void setZooKeeper();
    
    
    std::atomic_bool is_readonly {true};
    
    
    std::optional<bool> has_metadata_in_zookeeper;
    static constexpr auto default_zookeeper_name = "default";
    String zookeeper_name;
    String zookeeper_path;
    String replica_name;
    String replica_path;
    /** /replicas/me/is_active.
      */
    zkutil::EphemeralNodeHolderPtr replica_is_active_node;
    /** Is this replica "leading". The leader replica selects the parts to merge.
      * It can be false only when old ClickHouse versions are working on the same cluster, because now we allow multiple leaders.
      */
    std::atomic<bool> is_leader {false};
    InterserverIOEndpointPtr data_parts_exchange_endpoint;
    MergeTreeDataSelectExecutor reader;
    MergeTreeDataWriter writer;
    MergeTreeDataMergerMutator merger_mutator;
    MergeStrategyPicker merge_strategy_picker;
    /** The queue of what needs to be done on this replica to catch up with everyone. It is taken from ZooKeeper (/replicas/me/queue/).
     * In ZK entries in chronological order. Here it is not necessary.