**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.
const String & last_log_entry = *std::max_element(log_entries.begin(), log_entries.end());
                res.log_max_index = parse<UInt64>(last_log_entry.substr(strlen("log-")));
            }
            String log_pointer_str = zookeeper->get(fs::path(replica_path) / "log_pointer");
            res.log_pointer = log_pointer_str.empty() ? 0 : parse<UInt64>(log_pointer_str);
            auto all_replicas = zookeeper->getChildren(fs::path(zookeeper_path) / "replicas");
            res.total_replicas = all_replicas.size();
            for (const String & replica : all_replicas)
            {
                bool is_replica_active = zookeeper->exists(fs::path(zookeeper_path) / "replicas" / replica / "is_active");
                res.active_replicas += static_cast<UInt8>(is_replica_active);
                res.replica_is_active.emplace(replica, is_replica_active);
            }
        }
        catch (const Coordination::Exception &)
        {
            res.zookeeper_exception = getCurrentExceptionMessage(false);
        }
    }
}
void StorageReplicatedMergeTree::getQueue(LogEntriesData & res, String & replica_name_)
{
    replica_name_ = replica_name;
    queue.getEntries(res);
}
std::vector<PartMovesBetweenShardsOrchestrator::Entry> StorageReplicatedMergeTree::getPartMovesBetweenShardsEntries()
{
    return part_moves_between_shards_orchestrator.getEntries();
}
time_t StorageReplicatedMergeTree::getAbsoluteDelay() const