#3 Eliminate raw storage of return value in rpa::future

[Remi Chateauneu]

rpa::future stores the return value of a functor, in raw storage - this avoids to call the default constructor of the return value.
This might not be neveccsary because rpa::future is always used in conjunction of its functor and, all functors stores internally their return values. Therefore, it might be sufficient to store the const reference of the return value.
This will avoid some memory waste and a copy constructor.