Draft: Move "Didn't recognize cell, but circ stops here" into heartbeat.
When we looked, this was the third most frequent message at PROTOCOL_WARN, and doesn't actually tell us what to do about it. Now:
- we just log it at info
- we log it only once per circuit
- we report, in the heartbeat, how many times it happens, how many cells it happens with per circuit, and how long these circuits have been alive (on average).
Fixes the final part of #40400 (closed).