[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.*`
* WIP, started unordered_set<SensorId>
* struct SensorId. Works for cartographer without grpc.
* correct test
* SensorId in cartographer_grpc/
* clean up
* try to fix for trusty
* SensorId::operator==
* Ran clang-format.
Alexander Belyaev
Christoph Schütte
gaschler