they only add the context id to the tag selection of the underlying
messaging meachinsm.
We would like to enable an MTL to maintain its own context data
per-communicator. This way an MTL will be able to queue incoming eager
messages and rendezvous requests per-communicator basis.
The MTL will be allowed to override comm->c_pml_comm member,
since it's unused in pml_cm anyway.
This commit was SVN r24858.
fd17f20ed5