Not all nodes will get indexed on busy sites

If more nodes are added in between cron.php calls than the "Items to index per cron run". The problem lies in that indivudal nodes are not flagged as indexed, and the node module uses the last index/cron run timestamp to look for new nodes to index.

Example:
Cron run sets timestamp to 1000
200 nodes are added before next cron run all with timestamps between 1001 and 2000
Cron run executes finds 200 nodes to index with timestamps / change dates greater than 1000, but it only indexes the first 100. Cron then sets timestamp 2010.
Cron run executes again and find zero nodes with timestamps greater than 2010, even though only a 100 of the previously 200 found where indexed.

This patch creates a timestamp on the node records which indicates there last index, and updates these after indexing.