fix(PruneChangesBeforeJob): Various fixes (brought from 4.0 branch) (#23)
fix(PruneChangesBeforeJob): Various fixes (brought from 4.0 branch) (#23)

- fix(PruneChangesBeforeJob): Remove unused / old code that retrieved a flat list of IDs as this can cause the job to not work if there are a lot of DataChangeRecord items.
- fix(PruneChangesBeforeJob): Fix logic of priorTo so that it defaults to the last 3 months properly
- fix(PruneChangesBeforeJob): Change steps to 1 as it's technically doing this in 1 step now, this is to avoid an issue where totalSteps=0 can occur and the job won't requeue itself.
silverstripe-australia/silverstripe-datachange-tracker,silverstripe-australia/datachange-tracker