On the client side, we always use the configured packet size. This comes
with the risk of failing the handshake if the path doesn't support this
MTU. If the server sends a max_udp_payload_size that's smaller than this
size, we can safely ignore this: Obviously, the server still processed
the (fully padded) Initial packet, despite claiming that it wouldn't do
so.
On the server side, there's no downside to using 1200 bytes until we
received the client's transport parameters:
* If the first packet didn't contain the entire ClientHello, all we can
do is ACK that packet. We don't need a lot of bytes for that.
* If it did, we will have processed the transport parameters and
initialized the MTU discoverer.
