Should MPIX_Op_create_x take an int concurrent argument?

[raffenet]

In #6965, an additional argument was suggested (int concurrent) to inform MPI that a user-defined op is safe to execute concurrently. The suggestion is based on prior and upcoming work showcasing benefits of multithreading inside MPI for computation. See for reference https://dl.acm.org/doi/10.1145/2597652.2597658.

We can use this discussion to aggregate further references to published work, ultimately to finalize any design proposed for standardization.