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.
Because the thread that is actually terminating the SA and thus checking
it in again is not be the same thread that previously checked it out and
queued the termination job, the thread local SA on the bus has to be reset
manually, similar to how it is set in the job which is executing the job.
Tobias Brunner