The row-version chaining in Serializable Snapshot Isolation was still wrong.

The row-version chaining in Serializable Snapshot Isolation was still wrong.
On further analysis, it turns out that it is not needed to duplicate predicate
locks to the new row version at update, the lock on the version that the
transaction saw as visible is enough. However, there was a different bug in
the code that checks for dangerous structures when a new rw-conflict happens.
Fix that bug, and remove all the row-version chaining related code.

Kevin Grittner & Dan Ports, with some comment editorialization by me.