pageserver: full reconciliation of tenant locations (attached & secondary) on startup

[jcsp]

This is follow-on from #5299, but is a medium-term thing: it won't make sense to implement this until we do a more general revamp of how pageservers are updated about tenants, probably moving to a more reconciliation-loop like model.

Currently, our /re-attach API response only tells us about attached locations. When we have local data for a tenant that is omitted from this response, we have to choose between:

• A) detaching the tenant (and thereby dumping its local disk content)
• B) setting the tenant's state to secondary, and waiting for the control plane to detach it if that's what it wanted.

This would be much simpler if the /re-attach response included all state for tenant locations, but that would make it quite a lot bigger (approx 2x the size if it includes secondaries). We anticipate that switching to this model might be done at the same time as moving to a nicer gRPC-esque transport for this API, so that very large responses are not a source of risk.

[jcsp]

Support for secondaries was added in #6941