**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.
const String & from,
        bool fetch_part,
        ContextPtr query_context) override;
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    bool dropPartImpl(zkutil::ZooKeeperPtr & zookeeper, String part_name, LogEntry & entry, bool detach, bool throw_if_noop);
    
    
    bool checkFixedGranularityInZookeeper();
    
    void waitMutationToFinishOnReplicas(
        const Strings & replicas, const String & mutation_id) const;
    MutationCommands getFirstAlterMutationCommandsForPart(const DataPartPtr & part) const override;
    void startBackgroundMovesIfNeeded() override;