+ GRAS:
+ * Actually implement gras_datadesc_copy() so that we don't have to mimick
+ RL communication on top of SG since it's so uneffective.
+ It may also be used for inter-thread communication in RL, one day. [MQ]
+ * Use gras_datadesc_copy() to exchange messages on top of SG [MQ]
+ Allows to:
+ - improve message exchange performance on top of SG
+ - deprecate transport_plugin_sg.c:gras_trp_sg_chunk_send() & recv()