force submission touching during assignment import to use slave
force submission touching during assignment import to use slave

avoids a less-than-ideal query plan on large dbs

Change-Id: Ifef15fd1232323d42abb42b5f4f67114c52a8bd8
Reviewed-on: https://gerrit.instructure.com/123835
Tested-by: Jenkins
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Product-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
QA-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>

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