Probable change to HSS non-key storage layout

We have code for storing state for hidden services including IPT state, replay logs, and keys.

But the non-key state is a bit ad-hoc and unprincipled and has a strange filesystem layout (as seen in find on an arti state directory). See some of the notes in !1853 (merged). We want to improve this.

Probably this will be improved as a side effect of produding a new internal API as requested by #1163 (closed).