Added 'nodiscard' attribute to some member functions of Resources.
Added 'nodiscard' attribute to some member functions of Resources.

Added the `[[nodiscard]]` attribute to `Resources::pushReservation()`
and `Resources::popReservation()`, in order to prevent mistakes by
authors incorrectly assuming that these functions would modify the
resources objects in place.

Review: https://reviews.apache.org/r/71735/

kaysoky/mesos,Gilbert88/mesos,reneploetz/mesos,reneploetz/mesos,reneploetz/mesos,abudnik/mesos,reneploetz/mesos,abudnik/mesos,dforsyth/mesos,Gilbert88/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,kaysoky/mesos,abudnik/mesos,reneploetz/mesos,dforsyth/mesos,kaysoky/mesos,dforsyth/mesos,Gilbert88/mesos,dforsyth/mesos,abudnik/mesos,Gilbert88/mesos,kaysoky/mesos,dforsyth/mesos,Gilbert88/mesos,kaysoky/mesos,reneploetz/mesos,Gilbert88/mesos,kaysoky/mesos,reneploetz/mesos,dforsyth/mesos,dforsyth/mesos,abudnik/mesos,abudnik/mesos