std::shared_future can't be used for non-copyable types,
and std::future can't be captured in std::function context
because it's also non-copyable.

A solution is to used a shared_ptr of future, that can
be copied around and the future result can be moved out.

Reviewed-by: Alexandre Viau <alexandre.viau@savoirfairelinux.com>
Change-Id: I58a96dbeace446776b40fec2a10818a544f0b833