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.

[zbeekman]

This is still an issue, IMO, but I think there are bigger issues with sendget_by_ref captured elsewhere. Therefore, I'll leave as closed unless someone thinks I should re-open.