**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.
create_result = createLogEntryToMutatePart(
                        *part,
                        future_merged_part->uuid,
                        desired_mutation_version->first,
                        desired_mutation_version->second,
                        merge_pred.getVersion());
                    if (create_result == CreateMergeEntryResult::Ok ||
                        create_result == CreateMergeEntryResult::LogUpdated)
                        break;
                }
            }
        }
    }
    catch (...)
    {
        tryLogCurrentException(log, __PRETTY_FUNCTION__);
    }
    if (!is_leader)
        return;
    if (create_result != CreateMergeEntryResult::Ok
        && create_result != CreateMergeEntryResult::LogUpdated)
    {
        merge_selecting_task->scheduleAfter(storage_settings_ptr->merge_selecting_sleep_ms);
    }
    else
    {
        merge_selecting_task->schedule();
    }
}
void StorageReplicatedMergeTree::mutationsFinalizingTask()