Add support for various PostgreSQL settings

This adds support for:

• random_page_cost: https://gitlab.com/gitlab-com/infrastructure/issues/1630
• max_locks_per_transaction: https://gitlab.com/gitlab-com/infrastructure/issues/1559
• log_temp_files: https://gitlab.com/gitlab-com/infrastructure/issues/1557
• log_checkpoints: https://gitlab.com/gitlab-com/infrastructure/issues/1553

Right now we have some technical debt in #2192 (closed), where the geo-postgresql settings need to be updated each time we add new settings to the template.

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• 36feff65 - Add support for additional PostgreSQL settings

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added 1 commit

• 77c292b6 - Pg conf vars only if they exist.

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Workaround is to surround it by if block.

enabled an automatic merge when the pipeline for 77c292b6 succeeds

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• c489db0b - Missing if.

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merged

mentioned in commit ad4deced

mentioned in commit 066772cf

Picked into 9-1-stable, will go into 9.1.1.

@yorickpeterse Are these the values we use on GitLab.com? If not, can you please add the GitLab.com settings to https://about.gitlab.com/gitlab-com/settings/ along with a brief explanation of why we don't ship them as the defaults?

That process: ship GitLab.com settings unless there is a good reason not to, in which case we document it on the webpage with an explanation, should be our default. I'm wondering if there is a way to automate this: have a page that reads settings from omnibus and auto-populates a table... otherwise too many manual steps will make it hard to stay DRY and up to date.

@ernstvn Some of the settings we use for GitLab.com differ from the defaults shipped in Omnibus. For example, log_temp_files and log_checkpoints are not very useful for most users and so they're disabled by default. Others are environment specific.

Having said that, for some of these it should be fine to change the defaults so I'll submit an MR to Omnibus for that.

can you please add the GitLab.com settings to https://about.gitlab.com/gitlab-com/settings/ along with a brief explanation of why we don't ship them as the defaults?

I'm a little bit torn on this. For example, we use 110GB for the shared_buffers setting because once upon a time we thought that was a good idea. What would the practical benefit be of putting this on said page? It's not a setting you can just copy 1:1, instead you have to set it based on your needs/environment.

mentioned in merge request !1675 (merged)

Some of the defaults will be adjusted in https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1675

mentioned in issue gitlab-com/infrastructure#2142 (closed)

Addressing the bigger topic in https://gitlab.com/gitlab-com/infrastructure/issues/2142