[Mpi-22] Ordering guarantees and multiple threads

[Darius Buntinas]

I've just created a ticket:
  https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/144
to suggest that we strengthen the ordering guarantees between sends (and
receives) from different threads.

I think that the real question here is whether the potential benefit to
the user outweighs the potential for optimization in a multithreaded
implementation.

Please have a look at it and feel free to comment.

Thanks,
-d