Only wait_for_less_busy_worker in cluster mode

[joshuay03]

Description

Refs: #2079 and #2940

It looks like the wait_for_less_busy_worker logic also runs in single mode. This means that when a thread becomes available, the server unnecessarily delays handling the request as there are no other workers whose threads can pick it up instead.

This PR ensures wait_for_less_busy_worker only happens if puma is running in cluster mode.

Your checklist for this pull request

• I have reviewed the guidelines for contributing to this repository.
• I have added (or updated) appropriate tests if this PR fixes a bug or adds a feature.
• My pull request is 100 lines added/removed or less so that it can be easily reviewed.
• If this PR doesn't need tests (docs change), I added [ci skip] to the title of the PR.
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• All new and existing tests passed, including Rubocop.

[joshuay03]

Hmm the tests added in #2079 seems to be just a server test, asserting that requests are delayed within the same thread pool. I'll look into rewriting them.

[joshuay03]

I'll look into rewriting them.

Actually, @nateberkopec, does this patch make sense / will it be accepted? Thought I'd check first before going and rewriting tests.

[dentarg]

This makes sense to me, please go ahead 👍🏻

[dentarg]

@joshuay03 have you seen #3325?

[joshuay03]

@joshuay03 have you seen #3325?

Ah whoops I did not, I should really check open PRs before opening one 🤦🏽‍♂️ I'll close this in favour of that.