Superseding alerts does not work

[zachomedia]

Need to handle: If the alerts get processed out of order, superseding of alerts does not take place (i.e., the flag superseded is still false).

When processing a new alert, the alert service should check for alerts referencing the current alert. If yes, then supersede it.

An alternative is to have a cleanup cron job run every so often.

[zachomedia]

Implemented.

There is a race condition though, so rarely alerts are not superseded as they should be.