fix deadlock between stream items and conversations touching users
fix deadlock between stream items and conversations touching users

fixes CNVS-12621

test plan:
 * start a conversation between two users
 * reply to the conversation as one user
 * as the other user, refresh the dashboard; confirm the unread messages
   count went up

Change-Id: Ia9c58b9ce03b3a1befd5cfdb64774c4a73bedce9
Reviewed-on: https://gerrit.instructure.com/34734
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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