Requested by @andreimatei and @dhartunian

For the time being, we are going to work under the assumption that for a given tenant, either

• all SQL servers are in-process with KV,
• OR all SQL servers are out-of-process.

but not both.

As of v23.1, our main plan to enforce this assumption is via runbooks (i.e. tell users "don't do that").

However, we probably want a check in the product to prevent mistakes.

This could be done e.g. by looking at the sql instances/sessions table to see whether there's a session open.

Jira issue: CRDB-22187

Epic CRDB-26691