avoid a transaction (even on the slave) processing external feeds

fixes CNVS-16468

Change-Id: I1c5afb3351340e1cf8580143f1b722830a757412
Reviewed-on: https://gerrit.instructure.com/43332
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>