mhnsw: change storage format

instead of one row per node per layer, have one row per node.
store all neighbors for all layers in that row, and the vector itself too

it completely avoids searches in the graph table and
will allow to implement deletions in the future

sql/handler.cc

@@ -8088,8 +8088,7 @@ int handler::prepare_for_insert(bool do_create)
     return 1;
 
   /* Preparation for unique of blob's */
-  if (table->s->long_unique_table || table->s->period.unique_keys ||
-      table->hlindex)
+  if (table->s->long_unique_table || table->s->period.unique_keys)
   {
     if (do_create && create_lookup_handler())
       return 1;