pml/ob1: as per past RFC bring the inline send optimization to
MPI_Isend.

I filed an RFC for this optimization some time back. It is a
relatively simple optimization. If the data associated with an
MPI_Isend can be put on the wire without allocating an MPI_Request
then do so. In this case we can legally return omp_request_empty
which will correctly indicate that the request is complete and that is
was not cancelled (these are the only requirements on send requests).

cmr=v1.8.3:reviewer=bosilca