**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->getBytesOnDisk(); }, settings.select_sequential_consistency);
    return res;
}
void StorageReplicatedMergeTree::assertNotReadonly() const
{
    if (is_readonly)
        throw Exception(ErrorCodes::TABLE_IS_READ_ONLY, "Table is in readonly mode (replica path: {})", replica_path);
}
SinkToStoragePtr StorageReplicatedMergeTree::write(const ASTPtr & , const StorageMetadataPtr & metadata_snapshot, ContextPtr local_context)
{
    const auto storage_settings_ptr = getSettings();
    assertNotReadonly();
    const Settings & query_settings = local_context->getSettingsRef();
    bool deduplicate = storage_settings_ptr->replicated_deduplication_window != 0 && query_settings.insert_deduplicate;
    
    
    return std::make_shared<ReplicatedMergeTreeSink>(
        *this, metadata_snapshot, query_settings.insert_quorum.valueOr(0),
        query_settings.insert_quorum_timeout.totalMilliseconds(),
        query_settings.max_partitions_per_insert_block,
        query_settings.insert_quorum_parallel,
        deduplicate,
        local_context);
}
bool StorageReplicatedMergeTree::optimize(