Optimize caf_sendget_by_ref

[vehre]

Use MPI-Datatypes to and sophisticated communication functions to efficiently transfer data between two remote images.

[rouson]

Shall we mark this as wont-fix and close it?

[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.