MINOR: quic: Ping from Initial pktns before reaching anti-amplification limit

MINOR: quic: Ping from Initial pktns before reaching anti-amplification limit

There are cases where there are enough room on the network to send 1200 bytes
into a PING only Initial packets. This may be considered as the last chance
for the connection to complete the handshake. Indeed, the client should
reply with at least a 1200 bytes datagram with an Initial packet inside.
This would give the haproxy endpoint a credit of 3600 bytes to complete
the handshake before reaching the anti-amplification limit again, and so on.