ch4: default MPIR_CVAR_CH4_ROOTS_ONLY_PMI on

[hzhou]

Pull Request Description

Using ROOTS_ONLY_PMI significantly improves the large-scale init time. It is about time to make it default.

[skip warnings]

Author Checklist

• [x] Provide Description Particularly focus on why, not what. Reference background, issues, test failures, xfail entries, etc.
• [x] Commits Follow Good Practice Commits are self-contained and do not do two things at once. Commit message is of the form: module: short description Commit message explains what's in the commit.
• [ ] Passes All Tests Whitespace checker. Warnings test. Additional tests via comments.
• [x] Contribution Agreement For non-Argonne authors, check contribution agreement. If necessary, request an explicit comment from your companies PR approval manager.

[hzhou]

test:mpich/ch4/all

[hzhou]

test:mpich/ch4/ofi

[hzhou]

Thanks for the review @raffenet . The tests are not clean, but all of them are testing congestion issue that are not related to this PR. I'll merge now. If there are recurring testing issues, we'll address them afterward.