Make sure we handle (and send!) padding for INTRODUCE2 payloads

See section PROCESS_INTRO2 in rend-spec-v3.txt: there is optional padding at the end of a decrypted introduce2 body (currently line 1935). We should make sure that we accept this padding. We should also make sure that clients generate it when we are sending introduce1 messages, so as to avoid leaking information about the size of the actual message body.

@mikeperry This is a case of "extra data at the end of a message must be ignored", but I believe in this case it is correct and safe. Do you agree?