improve add_role_id_columns migration performance

Change-Id: I6550a53b2cde3ff9e5638c5af9c60ed77698e9b7
Reviewed-on: https://gerrit.instructure.com/44208
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Bryan Madsen <bryan@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>