merge enrollments in a transaction during user merge

so that a cursor can be used

Change-Id: Ic716f42418962290d5e897df17c82d81bb6516e0
Reviewed-on: https://gerrit.instructure.com/32708
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>