Lock writes during gitlab:setup task

What does this MR do and why?

Lock writes during gitlab:setup task

This helps protect against split-brain situations in case of bugs / mis-configurations for decomposed setups.

Lock writes should do nothing for single-database setups

Related issue: #393145 (closed)

Screenshots or screen recordings

Lock writes appear in newly setup databases (two databases configured):

main database	ci database

No lock writes in new setup database (single database configured):

How to set up and validate locally

1. Change your config/database.yml database: to temporary, eg. gitlabhq_development_temp. So you don't lose data in next step
2. Run bundle exec rake gitlab:setup

After you are done testing, you can restore your config/database.yml

MR acceptance checklist

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