don't load all external_feeds into memory at once

Change-Id: Ia83d96b781b679907000ae1db33b60afbd7461df
Reviewed-on: https://gerrit.instructure.com/39859
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>