Ensure stream ID reuse is properly accounted in dropped cells

In the Knock-Knock paper, they re-use stream IDs to create a covert channel against onion services: https://dblp.org/rec/conf/wcnc/ZhangCW0LS24.html

Depending on the timeline for service-side onions in arti w/ PoW support (and maybe also onionbalance), we should consider fixing this for the vanguards addon to pick up.