EXPLORATORY == DON'T MERGE THIS - Flatbuffer implementation

This is in early stage for now.

+ client/rust part 
  - integration with the `build.rs`
  - connector code has been adapted (binser is mostly unused)
  - I removed the tests in connector/unix.rs  // TO BE DISCUSS :)
    (I guess most of the functionnalities are defered to flatbuffers)

+ tansiv/cpp 
  - only on the test part
  - not integrated in the coodinator yet

+ cmake
  - integration with flatbuffer requires the sources to be located somewhere and pointed with `DFLATBUFFERS_SRC=`
    (I suspect we should use 2.0.0 to align with the rust flatbuffer runtime)

For #26 (closed)