Upstream protobuf removed scoped_ptr.h, which provided the scoped_ptr
that we depended on, in commit 67952fab2c766ac5eacc15bb78e5af4039a3d398
(67952fab2c).
This commit converts uses of scoped_ptr type to idiomatic C++11
std::unique_ptr, similar to the changes in the protobuf commit.
This has been tested on protobuf 3.0.0 (the version currently in Debian)
as well as the latest protobuf 3.6.1 release.
83c59e705f