Defer use of handshake transport parameters until the handshake is complete

[martinthomson]

When the client is sending 0-RTT, I think that it will start using the updated transport parameters from the server immediately after receiving them, rather than waiting for the handshake to complete.

We sort of agreed not to, mostly so that servers that buffer 0-RTT don't have to deal with the new limits in the handshake. That doesn't make a lot of sense, because the next 0-RTT will have to deal with those limits, but that's what the spec says.

We might be able to make the management of transport parameters simpler as a result of doing this.