Document how to design state queries (and writes)

[teor2345]

This needs to be done before #7738, but it's not a ticket template.

Everything that goes in the docs doesn't need to go in the ticket template, I can just link to the docs from the template.

This should document the typed column families from PR #8112.

[teor2345]

From PR #7972:

Document how to avoid this happening again in the state upgrade docs.

In particular, don't concurrently read and write the same keys in the same column family:

• in an upgrade, and
• in the sync.

If we need to do that, we should make the upgrade happen before syncing. (That is, block on the upgrade when the state is opened.)

And make sure the read and write methods have compatible conditions.

[teor2345]

The typed column families from PR #8112 might also help with some common database bugs.