Add some new code comments on the log compress/decompress

Signed-off-by: Jens Axboe <axboe@fb.com>
tp: move pthread_cond_signal() outside of lock

Signed-off-by: Jens Axboe <axboe@fb.com>
mutex: move pthread_cond_signal() outside of lock

Generally best practice to drop the lock before waking, if
possible.

Signed-off-by: Jens Axboe <axboe@fb.com>