Make persistent DB connections configurable

config/db.inc.php.dist

@@ -32,6 +32,10 @@ $rcmail_config['db_backend'] = 'db';
 // maximum length of a query in bytes
 $rcmail_config['db_max_length'] = 512000;  // 500K
 
+// use persistent db-connections
+$rcmail_config['db_persistent'] = TRUE;
+
+
 // you can define specific table names used to store webmail data
 $rcmail_config['db_table_users'] = 'users';