use timeout code in cassandra backfills
use timeout code in cassandra backfills

refs CNVS-48876
flag = none

also only summarize jobs from the current
shard.

TEST PLAN:
  1) run in production
  2) cassandra timeout rate in sentry drops
     dramatically

Change-Id: I318cb3d8d0d9da73801e1dc24bdab94cc0546807
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/236788
Reviewed-by: Jacob Burroughs <8ecea6e385af5cf9f53123f5ca17fb5fd6a6d4b2@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Ethan Vizitei <73eeb0b2f65d05a906adf3b21ee1f9f5a2aa3c1c@instructure.com>
Product-Review: Ethan Vizitei <73eeb0b2f65d05a906adf3b21ee1f9f5a2aa3c1c@instructure.com>

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