pmix: correctly split pmi messages

Thanks to @elenash for all the reviews
Merge pull request #245 from ggouaillardet/poc/pmi_slurm

pmix: correctly split pmi messages
dstore: remove unused variables
Per the PR and discussion on today's telecon, extend the process name definition as a two-field struct of uint32_t's down to the OPAL layer. This resolves issues created by prior commits that impacted both heterogeneous and SPARC support. This also simplifies the OMPI code base by removing the need for frequent memcpy's when transitioning between the OMPI/ORTE layers and OPAL.

We recognize that this means other users of OPAL will need to "wrap" the opal_process_name_t if they desire to abstract it in some fashion. This is regrettable, and we are looking at possible alternatives that might mitigate that requirement. Meantime, however, we have to put the needs of the OMPI community first, and are taking this step to restore hetero and SPARC support.
dummy commit
force Jenkins to test vs the master ...