BUG/MINOR: quic: drop multiple Retry on same connection

BUG/MINOR: quic: drop multiple Retry on same connection

Ensures that only a single Retry packet is handled by a QUIC haproxy
client per connection. This is mandated by RFC 9000. The first received
token should still be sufficient to validate the connection.

This change is applied directly in quic_rx_pkt_parse(). In case of
multiple Retry, packets are silently ignored, whether token is identical
or not.

This fix is particularly important to prevent a memory leak on several
elements, first <retry_token> member of quic_conn. This also concerns
elements from the TLS stack as initial encryption level would be
reinitialized needlessly.

Reported-by: Claude (ANT-2026-CJ4Z875H)
This must be backported up to 3.3.