**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.
cleanup_thread.wakeup();
            }
            else
                tryLogCurrentException(log, __PRETTY_FUNCTION__);
            /** This exception will be written to the queue element, and it can be looked up using `system.replication_queue` table.
              * The thread that performs this action will sleep a few seconds after the exception.
              * See `queue.processEntry` function.
              */
            throw;
        }
        catch (...)
        {
            tryLogCurrentException(log, __PRETTY_FUNCTION__);
            throw;
        }
    });
}
bool StorageReplicatedMergeTree::scheduleDataProcessingJob(BackgroundJobsAssignee & assignee)
{
    
    if (queue.actions_blocker.isCancelled())
        return false;
    
    ReplicatedMergeTreeQueue::SelectedEntryPtr selected_entry = selectQueueEntry();
    if (!selected_entry)
        return false;
    auto job_type = selected_entry->log_entry->type;
    
    if (job_type == LogEntry::GET_PART)
    {