ofi/common: cannot handle general sessions model

Looks like some world-model thinking got built into the ofi common code.
See sections 11.2 and 11.3 of the MPI 4.0 standard for a discussion of these two different methods for ```initializing``` MPI.

the sessions_init_twice test (https://github.com/open-mpi/ompi-tests/blob/master/ibm/sessions/sessions_init_twice.c) case fails when using OFI MTL currently.