**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.
throw;
    }
    
    
    storage.replica_is_active_node = zkutil::EphemeralNodeHolder::existing(is_active_path, *storage.current_zookeeper);
}
void ReplicatedMergeTreeRestartingThread::partialShutdown(bool part_of_full_shutdown)
{
    setReadonly(part_of_full_shutdown);
    ProfileEvents::increment(ProfileEvents::ReplicaPartialShutdown);
    storage.partial_shutdown_called = true;
    storage.partial_shutdown_event.set();
    storage.replica_is_active_node = nullptr;
    LOG_TRACE(log, "Waiting for threads to finish");
    storage.merge_selecting_task->deactivate();
    storage.queue_updating_task->deactivate();
    storage.mutations_updating_task->deactivate();
    storage.mutations_finalizing_task->deactivate();
    storage.cleanup_thread.stop();
    storage.part_check_thread.stop();
    
    {
        auto fetch_lock = storage.fetcher.blocker.cancel();
        auto merge_lock = storage.merger_mutator.merges_blocker.cancel();
        auto move_lock = storage.parts_mover.moves_blocker.cancel();
        storage.background_operations_assignee.finish();
    }
    LOG_TRACE(log, "Threads finished");
}
void ReplicatedMergeTreeRestartingThread::shutdown()