1) Java part:
Use java.util.logging with DEBUG/TRACE level in
generated JVppXXXImpl and Future/Callfacade impl classes
to log messages that are sent/received via JNI.

2) C part:
Investigate vpp logging mechanisms.
If there is no way to configure log levels, then enable logs just for DEBUG build.
Log messages sent/received via SHM.
VPP provide a way to enable api trace, so perhaps we could reuse some formatting logic.

Be careful to not influence performance much.