**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.
void checkTableCanBeDropped() const override;
    ActionLock getActionLock(StorageActionBlockType action_type) override;
    void onActionLockRemove(StorageActionBlockType action_type) override;
    
    
    bool waitForShrinkingQueueSize(size_t queue_size = 0, UInt64 max_wait_milliseconds = 0);
    
    struct Status
    {
        bool is_leader;
        bool can_become_leader;
        bool is_readonly;
        bool is_session_expired;
        ReplicatedMergeTreeQueue::Status queue;
        UInt32 parts_to_check;
        String zookeeper_path;
        String replica_name;
        String replica_path;
        Int32 columns_version;
        UInt64 log_max_index;
        UInt64 log_pointer;
        UInt64 absolute_delay;
        UInt8 total_replicas;
        UInt8 active_replicas;
        String last_queue_update_exception;
        
        String zookeeper_exception;
        std::unordered_map<std::string, bool> replica_is_active;
    };