session: init with the user requested thread level

[hzhou]

Pull Request Description

Previously we assume we can't support session with MPI_THREAD_MULTIPLE because user always can concurrently call MPI from different sessions. However, if we interpret the semantics of MPI thread level governs above sessions, then user can call MPI_Session_init with any thread level and we can just support it as usual if it is the first init. Later init will just inherit the already-set thread level.

[skip warnings]

Author Checklist

• [x] Provide Description Particularly focus on why, not what. Reference background, issues, test failures, xfail entries, etc.
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[hzhou]

test:mpich/ch4/most

[hzhou]

test:mpich/ch4/most