msgpack-rpc: Improve logging of msgpack-rpc messages

- Expose more logging control from the log.c module(get log stream and omit
  newlines)
- Remove logging from the generated functions in msgpack-gen.lua
- Refactor channel.c/helpers.c to log every msgpack-rpc payload using
  msgpack_object_print(a helper function from msgpack.h)
- Remove the api_stringify function, it was only useful for logging msgpack-rpc
  which is now handled by msgpack_object_print.
test: verify that msgpacks-rpc exceptions are working