First implementation of minimal Broker API

O.k. now I'm wondering what's the use of flatbuffers then?

IIRC the argument was that flabuffers' schema compiler would enable us to define the serializable classes in a more generic way. flatbuffers then takes care of generating the actual application code. Hence, we get two things out of it: 1) we don't have to write the tedious serialization code ourselves 2) a very efficient memory layout that gets serialization done at minimum cost (basically by avoiding it altogether in the first place). If both these features aren't used, I wonder if we would be better off removing the dependency and putting stuff into our own byte buffer implementation (which is not too hard to do anyways).