Memory usage: Investigate ReactiveCachingWorker

From https://gitlab.com/gitlab-org/gitlab-ce/issues/49703#note_94708776, this one tends to use a lot of RAM (https://dashboards.gitlab.net/d/sQ4GXgpik/memory-usage?orgId=1&from=now-24h&to=now):

https://log.gitlab.net/app/kibana#/discover?_g=h@1251ff0&_a=h@8c1b45b

Work still in progress [#<struct Sidekiq::BasicFetch::UnitOfWork queue="queue:reactive_caching", job="{\"class\":\"ReactiveCachingWorker\",\"args\":[\"MergeRequest\",19314491,\"compare_test_results\"],\"retry\":3,\"queue\":\"reactive_caching\",\"jid\":\"e349564550701b468c61846c\",\"created_at\":1542233712.2017295,\"enqueued_at\":1542234316.1521823}">]

It seems that calculating test reports seems to be slow:

Relates to this feature: https://docs.gitlab.com/ee/ci/junit_test_reports.html