travis: Force verification of DLOG macros

We use INFO as the default log level, which leaves code inside DLOG macros
unverified(currently it has compilation errors).

Decrease the log level on travis builds for automatic verification in PRs.
debug: Fix broken DLOG macro calls
debug: Improve debugging of msgpack-rpc requests

- Add the api_stringify function to display API objects
- Use api_stringify to display request arguments and return values in DLOG
  statements.