Refactor transport layer

[jean-roland]

Transport layer refactor to improve maintainability and modularity in prevision of adding a new transport.

Instead of compile-time specific code removal spread everywhere, files call functions that are swapped to dummy versions when the feature is removed.

This may increase slightly code size when removing the feature but makes it much easier to expand the number of transports.

• added a few "early return" pattern here and there

[cguimaraes]

If possible avoid the early returns, unless they are really needed.
If you still keep them, document them (including the reason).