**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 removePartFromZooKeeper(const String & part_name, Coordination::Requests & ops, bool has_children);
    
    void removePartFromZooKeeper(const String & part_name);
    
    void removePartsFromZooKeeper(zkutil::ZooKeeperPtr & zookeeper, const Strings & part_names,
                                  NameSet * parts_should_be_retried = nullptr);
    
    void removePartsFromZooKeeperWithRetries(const Strings & part_names, size_t max_retries = 5);
    void removePartsFromZooKeeperWithRetries(DataPartsVector & parts, size_t max_retries = 5);
    
    void removePartAndEnqueueFetch(const String & part_name);
    
    /** Execute the action from the queue. Throws an exception if something is wrong.
      * Returns whether or not it succeeds. If it did not work, write it to the end of the queue.
      */
    bool executeLogEntry(LogEntry & entry);
    
    
    MutableDataPartPtr attachPartHelperFoundValidPart(const LogEntry& entry) const;
    void executeDropRange(const LogEntry & entry);
    
    
    
    bool executeMetadataAlter(const LogEntry & entry);
    
    
    
    
    bool executeFetch(LogEntry & entry, bool need_to_check_missing_part=true);
    bool executeReplaceRange(const LogEntry & entry);