iterator: move auto-seek data in its own struct, add comments

iterator: move auto-seek data in its own struct, add comments

Further patches will add more fields related to this feature, so it
seems useful to group them under the same (anonymous) struct.

No functional changes intended.

Change-Id: I6ed922801cf5bd9001d388d7d8b050a6c8d01790
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1484
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>