_thingValues should take a Person, not an Identity

Summary: Currently, _thingValues takes an Identity. That made sense once upon a time, but has long since been left behind by events -- it just causes hassles with no benefit. It should take Person instead.

This is actually somewhat tricky to fix, since we can easily get the Identity Id, but the code currently expects to receive a full PublicIdentity. Does it actually need that, or could we replace the whole pipeline with an IdentityId?