minor: require instance to be constructed before we make a pool
minor: require instance to be constructed before we make a pool

mongodb/mongo-cxx-driver,mongodb/mongo-cxx-driver,acmorrow/mongo-cxx-driver,xdg/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver,acmorrow/mongo-cxx-driver,xdg/mongo-cxx-driver,mongodb/mongo-cxx-driver,xdg/mongo-cxx-driver,xdg/mongo-cxx-driver,acmorrow/mongo-cxx-driver