Introduce a UUID message type

Currently when UUID need to be part of a protobuf message, we use a byte array field for that. This has some drawbacks, especially when it comes to outputting the UUID in logs: To stringify the UUID field, we first have to create a stout UUID, then call .toString() of that one. It would help to have a UUID type in mesos.proto and provide a stringification function for it in type_utils.hpp.