fix(mqtt-api): Use LOG macro as defined by the developer guide

The GNUnets developer style guide https://gnunet.org/style recommends the
following:

    logging should be unified per-file by defining a LOG macro with the
    appropriate arguments, along these lines:
    #define LOG(kind,...) GNUNET_log_from (kind, "example-api",__VA_ARGS__)

The GNUnet-MQTT API was violating this best practies until now.
feat(fix-testcase): Fix MQTT service s.t. the single peer testcase passes

- adds some logging to mqtt-service
- fixes regex search call, the closure was NULL which lead to a silent "crash"

closes #8