Ensure circuit padding RTT estimate handes var cells/wide creates

The use_rtt_estimate field in the circuit padding machines lets machines offset the inter-packet delays by a middle-node estimated RTT value of packets that go from the middle to the exit/website.

We abort this measurement if we get two or more cells back-to-back in either direction, as this indicates that the half-duplex request/response circuit setup and RELAY_BEGIN sequence has finished.

However, if we switch to a multi-cell circuit handshake, then we will need to take that into account when measuring RTT.

If RELAY_EARLY is used only for the first cell of a multi-cell EXTEND2 payload, then we can just count time between RELAY_EARLIES. But the proposal currently says MAY, but not MUST for this behavior.