Use a separate interface for loggers.

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.

src/libcharon/plugins/sql/sql_logger.h

@@ -32,9 +32,9 @@ typedef struct sql_logger_t sql_logger_t;
 struct sql_logger_t {
 
 	/**
-	 * Implements bus_listener_t interface
+	 * Implements logger_t interface
 	 */
-	listener_t listener;
+	logger_t logger;
 
 	/**
 	 * Destry the backend.