Ensure circuit padding can be safely disabled from consensus
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.