**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 executeClonePartFromShard(const LogEntry & entry);
    /** Updates the queue.
      */
    void queueUpdatingTask();
    void mutationsUpdatingTask();
    /** Clone data from another replica.
      * If replica can not be cloned throw Exception.
      */
    void cloneReplica(const String & source_replica, Coordination::Stat source_is_lost_stat, zkutil::ZooKeeperPtr & zookeeper);
    
    void cloneMetadataIfNeeded(const String & source_replica, const String & source_path, zkutil::ZooKeeperPtr & zookeeper);
    
    void cloneReplicaIfNeeded(zkutil::ZooKeeperPtr zookeeper);
    ReplicatedMergeTreeQueue::SelectedEntryPtr selectQueueEntry();
    MergeFromLogEntryTaskPtr getTaskToProcessMergeQueueEntry(ReplicatedMergeTreeQueue::SelectedEntryPtr entry);
    bool processQueueEntry(ReplicatedMergeTreeQueue::SelectedEntryPtr entry);
    
    
    void startBeingLeader();
    void stopBeingLeader();
    /** Selects the parts to merge and writes to the log.
      */
    void mergeSelectingTask();
    
    void mutationsFinalizingTask();
    /** Write the selected parts to merge into the log,
      * Call when merge_selecting_mutex is locked.
      * Returns false if any part is not in ZK.
      */
    enum class CreateMergeEntryResult { Ok, MissingPart, LogUpdated, Other };