**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.
{
    
    
    
    
    
    std::lock_guard lock(current_zookeeper_mutex);
    if (zookeeper_name == default_zookeeper_name)
    {
        current_zookeeper = getContext()->getZooKeeper();
    }
    else
    {
        current_zookeeper = getContext()->getAuxiliaryZooKeeper(zookeeper_name);
    }
}
zkutil::ZooKeeperPtr StorageReplicatedMergeTree::tryGetZooKeeper() const
{
    std::lock_guard lock(current_zookeeper_mutex);
    return current_zookeeper;
}
zkutil::ZooKeeperPtr StorageReplicatedMergeTree::getZooKeeper() const
{
    auto res = tryGetZooKeeper();
    if (!res)
        throw Exception("Cannot get ZooKeeper", ErrorCodes::NO_ZOOKEEPER);
    return res;
}
zkutil::ZooKeeperPtr StorageReplicatedMergeTree::getZooKeeperAndAssertNotReadonly() const