PrimaryKeySegmentIndex can be corrupted with AgilityContext

[ScottDugas]

When run with an agile context, PrimaryKeySegmentIndex may delete entries for segments that don't end up getting merged, forcing future updates to use a query to do updates, defeating the purpose of the PrimaryKeySegmentIndex. This is most likely to happen if there is some sort of error, such as a timeout in a transaction after the PrimaryKeySegmentIndex gets merged.