fix manual locking in mysql used by delayed job singletons

refs CNVS-7032

Change-Id: I6fb05ca85a1cc14a0a7091d277e224b2e5be237e
Reviewed-on: https://gerrit.instructure.com/28574
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>