Language bindings need to map request messages with replies (and their cardinality).

Currently mapping is inferred from message naming.
If naming does not follow conventions,
Java generator/ binding compilation might fail with
misleading message.
Same probably applies to other bindings (Python/Lua), but moves failure to
runtime.

Improving vppapigen instead of Python/Java/Lua/.. generators
scales better.

API conventions used by language bindings, were summarized on the wiki:
https://wiki.fd.io/view/VPP/API_Concepts