Pseudonymizer stress test

This issue is to track the pseudonymizer stress test that occured on 2018-06-15 16:15 UTC.

We want to make sure the current pseudonymizer implementation is working at scale, using a production replica. This should output CSV files, for consumption by the meltano-extract-gitlab package.

If the extraction completes and is successful, we should gather samples of the data for the security review. See meltano/meltano#191 (comment 81747792)

The pseudonymizer is this MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5532/

The files needed are:

gitlab/lib/pseudonymizer/dumper.rb
gitlab/lib/pseudonymizer/options.rb
gitlab/lib/tasks/gitlab/db.rake

/cc @jschatz1 @stanhu