This also allows us to generate the log message only once for all
loggers that need it (avoids calls to custom printf specifier callbacks).
To update the log levels loggers can simply be registered again.
The new interface does not allow loggers to unregister themselves from
the bus. This allows us to use a rwlock_t for them.
The latter also means that loggers can now be called concurrently by
multiple threads.
Martin Willi