MPI 4.1 - opaque data handles and state preservation across sessions

[hppritcha]

Some wording went in to the MPI 4.1 standard that is potentially impactive on the way Open MPI currently handles MPI data types and Session finalize.

See https://github.com/open-mpi/ompi/issues/12193 and entry 7 in section B1.1 of the MPI 4.1 standard