New checklist

@Ericson2314: Updating with the plan I put up in #1316 (comment):

• Nix to latest release
  • Merged to master
  • Deployed to staging (hydra.ngi0.nixos.org)
  • Ensure Schema is correctly what master says, doesn't have have traced of old CA experiment
    • Put NON NULL back on BuildOutputs.path and BuildStepOutputs.path (this part of the migration back off older CA stuff was not done automatically, but the contentAddressed column was correctly dropped).
  • Deployed to prod (hydra.nixos.org)
• Prepare for CA derivation support with lower impact changes #1316 C++ changes without SQL schema changes to master
  • Deploy to staging hydra.ngi0.nixos.org (follow up: makemake: Stage Hydra PR 1316 infra#316)
  • merge to master
  • Test some more (how? how long?)
  • Deploy to prod (hydra.nixos.org)
• Allow building content-addressed derivations with hydra, minimally #1349 Rest of feature with schema changes!
  • Deploy to on staging (hydra.ngi0.nixos.org) with new ca-derivations experimental feature kept disabled
  • Merge to master
  • Evaluate cost of the migration adding column to prod (OK, hopefully?)
    • ALTER COLUMN DROP NOT NULL seems fine
  • Test some more (how? how long?)
  • Deploy to prod (hydra.nixos.org) with new ca-derivations experimental feature kept disabled
• Enable new ca-derivations experimental feature on staging (hydra.ngi0.nixos.org)
• Fix deep resolutions code in Hydra (and test?)
• Enable ca-derivations experimental feature on prod (hydra.nixos.org)
• Improve UX with CA derivations #875 Better UI, with more schema changes

Original description

Not clear yet how much of an impact this will have on the Hydra code base. The main consequence of CA derivations is that we don't know output paths in advance, but maybe that doesn't matter much for Hydra.