Fix skip list starting from protocol 22

[sisuresh]

Description

Fix the skip list. The values will still be incorrect until the ledger hash propagates within a month after the protocol vote. The docs and the XDR comments should be updated as well if we decide to accept this protocol change.

This PR is also meant to be a discussion on if we should care about fixing the skip list or not.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[graydon]

If this is all that's required, then I think we should absolutely do it! Though I am mildly concerned that we're somehow missing something and will wind up shipping another broken version :)