[quicwg/base-drafts] Need to prevent amplification attack for any case where the server's response is big (#1309)

[ekr <[e-mail]>]

S 4.4.3 says

```
If the server expects to generate more than three Handshake packets in response to an Initial packet, it SHOULD include a PATH_CHALLENGE frame in each Handshake packet that it sends. After receiving at least one valid PATH_RESPONSE frame, the server can send its remaining Handshake packets. Servers can instead perform address validation using a Retry packet; this requires less state on the server, but could involve additional computational effort depending on implementation choices.
```

However, there are two other cases where the server might generate >3 packets outside of the handshake:

* The application data protocol is a server speaks first protocol and it has a large banner it wants to send in 0.5 RTT
* The client is doing 0-RTT

There are ways to fix this, but it's not clear to me that the protocol actually documents them.

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