Use channel padding to obscure circuit setup
We could use the code from legacy/trac#16861 (moved) to pad more often during circuit setup to defend against circuit fingerprinting by someone who is watching the guard node. This would be somewhat complicated, as it would need special timers that could be called more often than once per second (or could chain off eachother), but it could be done.
On the client side, it could be done from any of the onionskin or circuit launching calls. On the relay side, we probably would need some logic around padding more often so long as we're still getting RELAY_EARLY cells, or similar.
It's also debatable in general as to if circuit fingerprinting is still successful if you're just watching the guard. If it's not, this may not be worth doing (since it won't protect anyone from a malicious or compromised guard).