Add flag to provide an optional barrier in MPI_Finalize.

Add flag to provide an optional barrier in MPI_Finalize.
This is meant to help for codes which can aggressively cleanup memory at finalization, while other processes still have to use it.
For example when attributes are attached to a local communicator, in SMPI this communicator may be cleaned up by another process in the end, and the attribute has to still be valid at this point.
This was an issue with PETSC, for example.