don't use COPY for find_in_batches during migrations
don't use COPY for find_in_batches during migrations

fixes CNVS-43369

it can cause the connection to swap, which confuses AR's advisory lock
protecting against concurrent migraitons

Change-Id: I0e75ffc76dbe7408221c9865b93d0561bcd46b5b
Reviewed-on: https://gerrit.instructure.com/161494
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Tested-by: Jenkins
Reviewed-by: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms