We are adding a new unique index format to support secondary apply with timestamps. The new format will remain compatible with the old format. This ticket encapsulates adding support for inserting into indexes created in the old format:

• Create a test that populates a unique index in the old format, then switches to the new format and starts inserting data. (Don’t try to hook up FCV yet - that comes later, so another mechanism will be required in local testing).
• Have the insert/update path identify when an old-version entry exists and remove it, prior to doing an insert.
• There should be no additional work to support secondaries, but they should be tested at this stage.