Need to implement persistent collectives

[hppritcha]

From the MPI 4.0 standard changelog

7. Sections 3.7.3, 3.9, 6.13, 8.8, and 8.9 on pages 70, 94, 276, 437, and 445.
Persistent collective communication MPI_fALLGATHERj. . . g_INIT including persistent
collective neighborhood communication MPI_NEIGHBOR_fALLGATHERj. . . g_INIT
was added to the standard.

There's already a PR to move mpix_allgather_init and friends into mpi namespace - #9065

[hppritcha]

closing as a dup of #9061