[trace] have to dump the buffer, trace events might use the container that is about...

[trace] have to dump the buffer, trace events might use the container that is about to be freed

details:
- this only affects simulators with process migration
- this might break the timestamps order in the trace file,
a better solution consists in cleaning out from the buffer
only the affected events

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@9327 48e7efb5-ca39-0410-a469-dd3cf9ba447f