discussion entries: simplify create_participants transaction
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 <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Product-Review: Braden Anderson <5d57265d16cfe2e0f2e6eb7f13e876bb378ef0f1@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>

roxolan/canvas-lms,AndranikMarkosyan/lms,leftplusrightllc/canvas-lms,redconfetti/canvas-lms,fronteerio/canvas-lms,juoni/canvas-lms,HotChalk/canvas-lms,juoni/canvas-lms,utkarshx/canvas-lms,redconfetti/canvas-lms,sdeleon28/canvas-lms,SwinburneOnline/canvas-lms,gbleydon/canvas-lms,eCNAP/CANVAS,instructure/canvas-lms,utkarshx/canvas-lms,utkarshx/canvas-lms,efrj/canvas-lms,sfu/canvas-lms,wye0220/shindig-canvas,sdeleon28/canvas-lms,utkarshx/canvas-lms,antoniuslin/canvas-lms,ShreniiNepal/shrenii_lmp,va7map/canvas-lms,ShreniiNepal/shrenii_lmp,fronteerio/canvas-lms,shidao-fm/canvas-lms,Jyaasa/canvas-lms,antoniuslin/canvas-lms,jay3126/canvas-lms,wrdsb/canvas-lms,shidao-fm/canvas-lms,jay3126/canvas-lms,djbender/canvas-lms,Rvor/canvas-lms,Rvor/canvas-lms,roxolan/canvas-lms,kyroskoh/canvas-lms,ShreniiNepal/shrenii_lmp,grahamb/canvas-lms,roxolan/canvas-lms,narigone/canvas,tgroshon/canvas-lms,va7map/canvas-lms,sfu/canvas-lms,eCNAP/CANVAS,samuels410/greatlakes-lms,fronteerio/canvas-lms,matematikk-mooc/canvas-lms,dgynn/canvas-lms,juoni/canvas-lms,sfu/canvas-lms,skmezanul/canvas-lms,efrj/canvas-lms,efrj/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,redconfetti/canvas-lms,matematikk-mooc/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,narigone/canvas,sdeleon28/canvas-lms,shidao-fm/canvas-lms,matematikk-mooc/canvas-lms,samuels410/greatlakes-lms,whalejasmine/canvas-lms,SwinburneOnline/canvas-lms,jay3126/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms,wrdsb/canvas-lms,juoni/canvas-lms,whalejasmine/canvas-lms,leftplusrightllc/canvas-lms,snehakachroo1/LSI_replica,jay3126/canvas-lms,SwinburneOnline/canvas-lms,tgroshon/canvas-lms,instructure/canvas-lms,kyroskoh/canvas-lms,eCNAP/CANVAS,Rvor/canvas-lms,leftplusrightllc/canvas-lms,tgroshon/canvas-lms,matematikk-mooc/canvas-lms,Jyaasa/canvas-lms,wrdsb/canvas-lms,wye0220/shindig-canvas,instructure/canvas-lms,AndranikMarkosyan/lms,redconfetti/canvas-lms,dgynn/canvas-lms,Jyaasa/canvas-lms,gbleydon/canvas-lms,wye0220/shindig-canvas,antoniuslin/canvas-lms,greyhwndz/canvas-lms,leftplusrightllc/canvas-lms,HotChalk/canvas-lms,AndranikMarkosyan/lms,dgynn/canvas-lms,whalejasmine/canvas-lms,kyroskoh/canvas-lms,gbleydon/canvas-lms,greyhwndz/canvas-lms,skmezanul/canvas-lms,samuels410/greatlakes-lms,narigone/canvas,instructure/canvas-lms,instructure/canvas-lms,samuels410/greatlakes-lms,snehakachroo1/LSI_replica,va7map/canvas-lms,sdeleon28/canvas-lms,Jyaasa/canvas-lms,Rvor/canvas-lms,djbender/canvas-lms,narigone/canvas,va7map/canvas-lms,grahamb/canvas-lms,greyhwndz/canvas-lms,antoniuslin/canvas-lms,AndranikMarkosyan/lms,grahamb/canvas-lms,roxolan/canvas-lms,grahamb/canvas-lms,ShreniiNepal/shrenii_lmp,instructure/canvas-lms,fronteerio/canvas-lms,wrdsb/canvas-lms,shidao-fm/canvas-lms,whalejasmine/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,efrj/canvas-lms,HotChalk/canvas-lms,sfu/canvas-lms,kyroskoh/canvas-lms,tgroshon/canvas-lms,djbender/canvas-lms,greyhwndz/canvas-lms,SwinburneOnline/canvas-lms,eCNAP/CANVAS,djbender/canvas-lms,dgynn/canvas-lms,sfu/canvas-lms,skmezanul/canvas-lms,HotChalk/canvas-lms,snehakachroo1/LSI_replica,skmezanul/canvas-lms,snehakachroo1/LSI_replica