* 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.
Context: The `::grpc::ClientContext` represents a single RPC on the client side. Therefore it is illegal to share the same context between two different RPC invocations.