Changed MongoCursor constructor to native

This would get rid of the warning: ‘void HPHP::c_MongoCursor_ni___construct(HPHP::CObjRef, const HPHP::Object&, const HPHP::String&, const HPHP::Array&, const HPHP::Array&)’ defined but not used [-Wunused-function] in the current master branch.