Large synchrony EVENTS table leads to OOM during XML Site Backup

We use a <composite-id> type in hibernate for the EVENTS table (because synchrony has two columns as a composite id). However, hibernate treats entities with composite ids differently to entities with an id field. Specifically, hibernate when being asked what the id of an entity is (which is what we do to get a handle during export), it returns the entire entity object as the id.

We keep around these "transient" ids in our xml export to make sure we don't duplicate etc. The code that uses these ids assume that they are small.

We will have to re-implement it with a composite id class, and hopefully, that will fix the issue.