Iteration behavior changes depending on the log worker thread work

[Tpt]

In the following test, the presence or abscence of the db.process_commits call changes the iteration behavior of the last iter.prev() call.

If db.process_commit is never called, the last iter.prev() returns the key [0] as exepected, if db.process_commit is called before iterating, it returns None.

This test is doing both forward and backward iterations, I don't know if we want to support both on the same iterator...

	#[cfg(feature = "instrumentation")]
	#[test]
	fn test_process_commit_in_the_middle_of_iter() {
		let tmp = tempdir().unwrap();
		let mut options = Options::with_columns(tmp.path(), 1);
		options.columns[0].btree_index = true;
		options.always_flush = true;
		options.with_background_thread = false;

		let db = Db::open_or_create(&options).unwrap();
		db.commit::<_, Vec<u8>>(vec![(0, vec![0], Some(vec![0]))]).unwrap();
		db.process_commits().unwrap();
		let mut iter = db.iter(0).unwrap();
		iter.seek_to_last().unwrap();
		assert_eq!(iter.prev().unwrap(), Some((vec![0], vec![0]))); // State = 0
		assert_eq!(iter.next().unwrap(), None); // State = End
		assert_eq!(iter.prev().unwrap(), Some((vec![0], vec![0]))); // Fails because is None, work properly if process_commit is not called
	}

Changelog: 2022-10-24: Updated the bug description, the bug happens even if process_commits is called before iterating.