Use our own global variable for the FlexiHash instance

Instead of using an attribute inside $wpdb.

db-hash.php

@@ -3,17 +3,17 @@
 require_once(dirname(__FILE__) . '/flexihash.php');
 require_once( ABSPATH . 'wp-config.php' );
 
-$wpdb->hash_map = new Flexihash(null, R2DB_FLEXIHASH_REPLICAS);
+$r2db_hash_map = new Flexihash(null, R2DB_FLEXIHASH_REPLICAS);
 
 /* Hashing function to map blogs to databases.
  *
  * Implements a consistent hashing scheme using Flexihash.
  */
 function noblogs_db_callback($query, $wpdb) {
-  $wpdb_hash = $wpdb->hash_map;
+  global $r2db_hash_map;
   if (preg_match("/^{$wpdb->base_prefix}(\d+)_/", $wpdb->table, $matches)) {
     $blog_id = $matches[1];
-    return $wpdb_hash->lookup($blog_id);
+    return $r2db_hash_map->lookup($blog_id);
   }
 }
 $wpdb->add_callback('noblogs_db_callback');