perf: simplify join order to use multiple order keys instead of string.

[TrevorBergeron]

Change-Id: I8c37e9296b2e4e0ea87f6a7e836d48988d161d37

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Fixes #<issue_number_goes_here> 🦕

[TrevorBergeron]

Question from @tswast

Can we remove string_encoded now? Or is it still used elsewhere?

We still use string_encoded for concat. I can remove that next, but it will be a bit tricky, so leaving out of this pr.