Presently, when reading a descriptor, we validate its internal consistency with the ValidateSelf phase and its referential consistency with the ValidateCrossReferences phase. The problem is that if the descriptor has corrupt back-references, validation will fail and render the table unavailable even when the descriptor is leased just to run a query. Back-references only come into play when dropping things in DDL statements. For this reason we should split ValidateCrossReferences into two distinct phases and only validate forward references on reads.

Or, at least, only in reads for leasing for a query, make validation usage aware. Also, perhaps validate back-references, but log errors instead of returning them.

Jira issue: CRDB-18145