Modified use of instance in ITTI messages to have a coherent use of it. It always refers to the same unit (eNB or UE) in all tasks or sub-tasks. This will allow filtering on instance number in itti_analyser.

git-svn-id: http://svn.eurecom.fr/openair4G/trunk@4371 818b1a75-f10b-46b9-bf7c-635c3b92a50f