add a singleton to attachment thumbnail processing job
add a singleton to attachment thumbnail processing job

should help avoid duplicate thumbnail insertion race conditions

closes #CNVS-3065

Change-Id: If38ff0103ec24d6c20dcfeb5ac8d30ddec6d8aff
Reviewed-on: https://gerrit.instructure.com/92156
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

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