**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.
{
    bool old_val = false;
    bool became_readonly = storage.is_readonly.compare_exchange_strong(old_val, true);
    
    if (became_readonly && on_shutdown)
        return;
    if (became_readonly)
        CurrentMetrics::add(CurrentMetrics::ReadonlyReplica);
    
    if (on_shutdown)
        CurrentMetrics::sub(CurrentMetrics::ReadonlyReplica);
}
void ReplicatedMergeTreeRestartingThread::setNotReadonly()
{
    bool old_val = true;
    
    
    
    if (storage.is_readonly.compare_exchange_strong(old_val, false) && !first_time)
        CurrentMetrics::sub(CurrentMetrics::ReadonlyReplica);
}
}