Right now, to "disable" circuit padding, we have to set consensus parameters circpad_global_allowed_cells=1 and circpad_global_max_padding_pct=1, which actually only sets a limit of 1% of our traffic to be padding. Additionally, each padding cell that is *not* sent because this limit is reached causes an info log line to be written. We need another way to fully disable padding, for A/B performance testing. And we should rate limit that info log so relays logging at info don't print a line for every single padding cell they don't send.