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.
