Batching/Throttling support for notify

The attached patch implements batching/throttling support for Notify.

Many of us run sites on shared servers with an outgoing message limit. The current version of notify sends all of its notifications out at once, with no support for batching them, so at a certain point the module will run afoul of the outgoing message limit. I developed this because I ran into the limit on one of my servers. It is currently in service on that production box and is working well there.

This patch changes the database; instead of using notify_send_last as a drupal variable, that information is stored on each row in the notify table. That way, a proper query can be done and only part of the table notified at a time. There is an addition to the settings page to specify how many should be sent on each cron run; the default is 5000 (or essentially unlimited). It's up to you to set up the combination of your cron schedule and the value you choose here to make sure you don't hit your host's outgoing message limit.

The database update is required before the updated version can work correctly. For that reason, sending is disabled until the update.php is run.

If you have any questions or would like anyting finessed in this patch, I'll be happy to help out.

Thanks,
-Randy