What is the Problem Being Solved?

In the priceFeed coreEval, we'll be replacing the scaledPriceAuthority vats for all currencies.

Description of the Design

The scaledPriceAuthority vats should start after the corresponding priceFeeds have been replaced, and should leave a marker behind in the promiseSpace to gate the running of the auction and vault upgrades.

Security Considerations

None

Scaling Considerations

This is part of an effort to address #8400.

Test Plan

we'll use a3p-integration and various testnets to verify that the upgrade leaves us with working vats and auctions.