logthrdestdrv: Use log_queue_backlog_*()

Instead of using log_msg_ack() and friends, use the log_queue_backlog
family of functions to accept or drop a message, or to rewind the queue.

All users of LogThreadedDestDriver updated, and now they all use the
same mechanism.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>