Add an index for uid, tid on install

Hi,

I've noticed that if there are a LOT of users on the site and a number of calls are made to get the current points of a user, it can get very slow. On closer inspection, I saw that there was no index created on the uid, tid pair in the userpoints table.

So an update would be (I can provide in patch format if that is preferred):

// To sigify that this is a drupal 6 update
function userpoints_update_6009() {
  $ret = array();
  db_add_index($ret, 'userpoints', 'uid_tid', array('uid', 'tid'));
  return $ret;
}

I'm not sure if a similar index in needed in userpoints_txn yet but I noticed a significant speed bump upon doing this change.