**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.
}
            }
            for (auto & future : remove_futures)
            {
                auto response = future.get();
                if (response.error == Coordination::Error::ZOK || response.error == Coordination::Error::ZNONODE)
                    continue;
                if (Coordination::isHardwareError(response.error))
                {
                    success = false;
                    continue;
                }
                throw Coordination::Exception(response.error);
            }
        }
        catch (Coordination::Exception & e)
        {
            success = false;
            if (Coordination::isHardwareError(e.code))
                tryLogCurrentException(log, __PRETTY_FUNCTION__);
            else
                throw;
        }
        if (!success && num_tries < max_retries)
            std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    }
    if (!success)
        throw Exception(ErrorCodes::UNFINISHED, "Failed to remove parts from ZooKeeper after {} retries", num_tries);
}
void StorageReplicatedMergeTree::removePartsFromZooKeeper(