Optimize caf_send_by_ref

[vehre]

Use MPI-Datatypes to reduce the network traffic needed by a send_by_ref.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.