Add SQL code standard for casting OIDs to bigint

[seancolsen]

This PR is stacked on #3633.

This is a small dev-docs PR which adds a SQL code standard that I don't think anyone will disagree with. The content in the PR should be self-explanatory. I noticed that we have many places throughout the SQL code that violate this standard. I don't think it's worth fixing that code now. But I wanted to make this PR to alert other devs to this problem so that we can avoid it going forward.

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[mathemancer]

Nice catch!