[Internal hdrs RFC](https://github.com/googlecartographer/rfcs/blob/master/text/0003-internal-headers.md)
`LocalTrajectoryUploader` implementation was hidden in the anonymous namespace in `local_trajectory_uploader.cc` to remove dependency from handlers/ in the header. The follow-up PR will include splitting the files into `local_trajectory_uploader_interface.h` & `internal/local_trajectory_uploader.*`
Alexander Belyaev