Precise details of HS circuit dirtiness algorithm

C Tor treats an HS circuit as "too old" if it hasn't been "used" for MaxCircuitDirtiness. A "too old" circuit won't be used for new streams.

Does "used" mean (i) some data was sent on some stream or (ii) a new stream was created?

Arti's current behaviour (!1287 (merged)) is along the lines of (ii). Does this need to change?

MR discussion on this point: !1287 (comment 2914876) Previous ticket: #913 (comment 2914433)