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.
This avoids deadlocks caused by extensive listener_t implementations
which might want to acquire a lock which is currently held by another
thread wanting to log messages. Since the latter requires that thread
to acquire the same lock the initial thread currently holds this
previously resulted in a deadlock.
With this change logging messages does not require threads to acquire
the main lock in bus_t and thus avoids the deadlock.
Tobias Brunner