transition when we send our first padding packet, not on received

The RP circpad machine, on the relay side, should transition as soon as a padding cell is sent to the client according to the code comment. The code currently transitions on receiving a padding cell, not sending.

Copied from: https://github.com/torproject/tor/pull/1167

Gaba, I think this is Sponsor 2, because it's a circuit padding bug.

Mike or asn, please add tags.