- UPstream::Request wrapping class provides an opaque wrapper for
  vendor MPI_Request values, independent of global lists.

ENH: support for MPI barrier (blocking or non-blocking)
ENH: support for MPI sync-send variants

STYLE: deprecate waitRequests() without a position parameter

- in many cases this can indicate a problem in the program logic since
  normally the startOfRequests should be tracked locally.