discussion entries: simplify create_participants transaction

fixes CNVS-12567, CNVS-14786

test plan:
  * create discussion entries in a topic with multiple participants
  * verify that topic unread counts and message unread statuses
      are maintained correctly
  * if possible, load test simultaneous entry creation
      and verify that no deadlock occurs

Change-Id: I9c5ce430d9c7914648e5920bd317949fe15f22f3
Reviewed-on: https://gerrit.instructure.com/39318
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>
QA-Review: August Thornton <august@instructure.com>