Changes a node's changed timestamp to 'now' to force reindexing.
$nid: The node ID of the node that needs reindexing.
function search_touch_node($nid) {
db_update('search_dataset')
->fields(array(
'reindex' => REQUEST_TIME,
))
->condition('type', 'node')
->condition('sid', $nid)
->execute();
}