Do a proof of concept of 2-stage Symmetric FCV, ideally where:

• All metadata changing steps on upgrade happen on the target FCV.
• The featureCompatibilityVersion Server parameter and FCV document report using the new API.
• There is a way to crude way to persist where setFCV is (instead of today where it must guess based on the FCV).

It does not need to handle stepdowns, interruptions, errors, etc..

The purpose is to flag potential difficulties early and help in drafting the final design.