Denial of service in quinn-proto when using `Endpoint::retry()`

Quinn is a pure-Rust, async-compatible implementation of the IETF QUIC transport protocol. As of quinn-proto 0.11, it is possible for a server to accept(), retry(), refuse(), or ignore() an Incoming connection. However, calling retry() on an unvalidated connection exposes the server to a likely panic in the following situations: 1. Calling refuse or ignore on the resulting validated connection, if a duplicate initial packet is received. This issue can go undetected until a server's refuse()/ignore() code path is exercised, such as to stop a denial of service attack. 2. Accepting when the initial packet for the resulting validated connection fails to decrypt or exhausts connection IDs, if a similar initial packet that successfully decrypts and doesn't exhaust connection IDs is received. This issue can go undetected if clients are well-behaved. The former situation was observed in a real application, while the latter is only theoretical.