Add stuff to make compute tasks schedulable.

Each time a compute task ends, we decrement a counter for the tasks
that are the destination of a transfert from the finishing task. When
this counter reaches 0, the task is SD_SCHEDULABLE (All compute task
parents are in the SD_DONE state).

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@7886 48e7efb5-ca39-0410-a469-dd3cf9ba447f