QUIC: path aware in-flight bytes accounting.

QUIC: path aware in-flight bytes accounting.

On-packet acknowledgement is made path aware, as per RFC 9000, Section 9.4:
    Packets sent on the old path MUST NOT contribute to congestion control
    or RTT estimation for the new path.

To make this possible in a single congestion control context, the first packet
to be sent after the new path has been validated, which includes resetting the
congestion controller and RTT estimator, is now remembered in the connection.
Packets sent previously, such as on the old path, are not taken into account.

Note that although the packet number is saved per-connection, the added checks
affect application level packets only.  For non-application level packets,
which are only processed prior to the handshake is complete, the remembered
packet number remains set to zero.