Periodically clean up the notificationinstance table

Problem Definition

On JIRA instances that heavily use mail handlers, the notificationinstance table can grow a lot, which ends up causing performance issues - specially since it appears that JIRA does a full table scan frequently. On Oracle DBs, this can be costly and degrade server performance.

Suggested Solution

We should delete older entries from the table, similarly to what we did with the NotificationInstanceKiller that we implement as an upgrade task after JIRA 5.2 (see this page for more details). Also, we could add an index in the database for this table, to improve performance when searching on it.