MINOR: quic: Do not try to accept a connection more than one time

MINOR: quic: Do not try to accept a connection more than one time

We add a new flag to mark a connection as already enqueued for acception.
This is useful for 0-RTT session where a connection is first enqueued for
acception as soon as 0-RTT RX secrets could be derived. Then as for any other
connection, we could accept one more time this connection after handshake
completion which lead to very bad side effects.

Thank you to Amaury for this nice patch.