core: add version and structure to LUO json payload

core: add version and structure to LUO json payload

We might want to add more state to the LUO session json payload,
so add a version (to allow clean compat breaks if needed) and nest
the current fdstore contents under a 'units' object, so that more
top-level data can be added in the future without breaking
backward compatibility.

Follow-up for 257c35c1a3936f53b80f16397a6909f4cd81124d