Make last_processed_object_id be the last ID of fetched objects

[felipeelia]

Description of the Change

This PR is an attempt to fix the problem outlined in #2755. When the last object of a batch is skipped, the last_processed_object_id isn't properly updated. That is easier to test when indexing only one document at a time (--no-bulk), as the only ID in the queue is the one that was skipped.

Closes #2755

Verification Process

Add a code like this to the codebase:

add_filter(
	'ep_item_sync_kill',
	function( $skip, $object ) {
		return ( 2958 === $object->ID ) ? true : $skip;
	},
	10,
	2
);

And run wp elasticpress index --setup --no-bulk --show-errors --yes

Changelog Entry

Fixed: Sync process with skipped objects.

Credits

Props @juliecampbell @felipeelia

[felipeelia]

Attaching a zip here, to make things easier for those who can/want to test it: elasticpress.zip

[tott]

Looks good to me