ensure correct shard for discussion entry participants
ensure correct shard for discussion entry participants

test plan
 - have a user on another shard
 - it should mark the entry as read on the correct shard

fixes VICE-1999
flag=none

Change-Id: I56bf00d8f69c10a5db7821d22bbc868dfd7de3ee
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/272606
Reviewed-by: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>
QA-Review: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>
Product-Review: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms