send two ClientHello packets, potentially with different sizes

[marten-seemann]

This serves three purposes:

1. It protects against loss of the ClientHello.
2. It opens up the amplification window.
3. It can be used for detecting the Path MTU very early during the connection.

Number 3 is most useful if the server latches onto that larger MTU. It probably makes the most sense to make the first packet larger than the second one, since that increases the likelihood that the first ACK will be for that packet.

We need to check what happens if one of these is packets is lost. Obviously, this should not trigger a retransmission. I think that our retransmission logic ensures that, but I'm not 100% sure.