**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.
{
    LogEntryPtr & entry = selected_entry->log_entry;
    return queue.processEntry([this]{ return getZooKeeper(); }, entry, [&](LogEntryPtr & entry_to_process)
    {
        try
        {
            return executeLogEntry(*entry_to_process);
        }
        catch (const Exception & e)
        {
            if (e.code() == ErrorCodes::NO_REPLICA_HAS_PART)
            {
                
                LOG_INFO(log, fmt::runtime(e.displayText()));
            }
            else if (e.code() == ErrorCodes::ABORTED)
            {
                
                LOG_INFO(log, fmt::runtime(e.message()));
            }
            else if (e.code() == ErrorCodes::PART_IS_TEMPORARILY_LOCKED)
            {
                
                LOG_INFO(log, fmt::runtime(e.displayText()));