**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 ASTPtr &,
    const StorageMetadataPtr &,
    const ASTPtr & partition,
    bool final,
    bool deduplicate,
    const Names & deduplicate_by_columns,
    ContextPtr query_context)
{
    
    
    auto table_lock = lockForShare(query_context->getCurrentQueryId(), query_context->getSettingsRef().lock_acquire_timeout);
    assertNotReadonly();
    if (!is_leader)
        throw Exception("OPTIMIZE cannot be done on this replica because it is not a leader", ErrorCodes::NOT_A_LEADER);
    auto handle_noop = [&] (const String & message)
    {
        if (query_context->getSettingsRef().optimize_throw_if_noop)
            throw Exception(message, ErrorCodes::CANNOT_ASSIGN_OPTIMIZE);
        return false;
    };
    auto zookeeper = getZooKeeperAndAssertNotReadonly();
    const auto storage_settings_ptr = getSettings();
    auto metadata_snapshot = getInMemoryMetadataPtr();
    std::vector<ReplicatedMergeTreeLogEntryData> merge_entries;