fix a memleak coming from cyclic references in smart pointers

fix a memleak coming from cyclic references in smart pointers

Before that change, the SMPI extension of actors got a smart reference
onto the S4U actor. That prevented S4U actors from being garbage
collected, since a SMPI extension is freed (and its smart references
released) only when the S4U actor is destroyed.

The change of this commit simply replace smart references with dumb
pointers to break the deadlock in the reference cycle, so that s4u
actors get collected after use.