tor-hsclient: Expire old IPT data

If we don't use it for a whole, the whole of Data will be expired by the new machinery in !1290 (merged). However, we also need to expire individual IPTs.

This bug doesn't seem to have had a relevant TODO.

I have verified ad-hoc that HS connections still work afterwards, modulo #918 (closed). Testing is more complicated, and left for a TODO HS.