**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.
{
    UInt64 res = 0;
    foreachActiveParts([&res](auto & part) { res += part->rows_count; }, settings.select_sequential_consistency);
    return res;
}
std::optional<UInt64> StorageReplicatedMergeTree::totalRowsByPartitionPredicate(const SelectQueryInfo & query_info, ContextPtr local_context) const
{
    DataPartsVector parts;
    foreachActiveParts([&](auto & part) { parts.push_back(part); }, local_context->getSettingsRef().select_sequential_consistency);
    return totalRowsByPartitionPredicateImpl(query_info, local_context, parts);
}
std::optional<UInt64> StorageReplicatedMergeTree::totalBytes(const Settings & settings) const