Major bug fix in SURF.

In MSG, tasks that were transfered multiple times or whose state was changed did leave some variables in the linear system. Those variables were removed only when the corresponding tasks was destroyed. So sometimes, the result was correct (as in the comparison with masterslave3 of the old MSG).

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@847 48e7efb5-ca39-0410-a469-dd3cf9ba447f