Often live traces are removed even though the archived trace
doesn't exist. This commit checkes the existence strictly.

commit 10456b1e9240886432f565dd17689080bbb133b9
Merge: 312c1a9bdf8 a5f46278
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Thu Nov 29 14:33:21 2018 +0900

    Merge branch 'master-ce' into add-counter-for-trace-chunks

commit 312c1a9bdf8efc45c3fed5ff50f05cc589bbb4ed
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Wed Nov 28 20:06:18 2018 +0900

    Fix coding offence

commit e397cc2ccc1b2cf7f8b3558b8fa81fe2aa0ab366
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Wed Nov 28 14:40:24 2018 +0900

    Fix tracking archive failure

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

Integrate two workers into one ArchiveTraceWorker with pipeline_background queue. This queue takes loqer precedence than pipeline_default.

EE: Trace as artifacts (FileStorage and ObjectStorage)

Closes #4171

See merge request gitlab-org/gitlab-ee!4258