deletion executor <-> upload queue race

[skyzh]

as yet another observation at #9844, deletion that gets persisted in the deletion list might be executed after tenant reattach, and we should ensure uploads of the same file happen after it.

[jcsp]

I think the fix here is going to be to avoid re-using the same generation, rather than trying to remember this stuff across Tenant lifetimes. This might be pretty simple for ancestor branch detach if we make the detach API carry a new generation to be used in the tenant constructed after restart.