Rename the conflict types for the origin differ cases.

Rename the conflict types for the origin differ cases.

The conflict types 'update_differ' and 'delete_differ' indicate that a row
to be modified was previously altered by another origin. Rename those to
'update_origin_differs' and 'delete_origin_differs' to clarify their
meaning.

Author: Hou Zhijie
Reviewed-by: Shveta Malik, Peter Smith
Discussion: https://postgr.es/m/CAA4eK1+HEKwG_UYt4Zvwh5o_HoCKCjEGesRjJX38xAH3OxuuYA@mail.gmail.com