Packet numbers now start at 0 for every packet number spaces.
Furthermore, an ACK frame now only acknowledges packets in the packet
number space it was sent in.
There's no need for a check if more than protocol.MaxTrackedSentPackets
packets were sent. There are certain situations where we allow (via
SendingAllowed()) sending of more packets, and we shouldn't throw an
error when the session then actually sends these packets.
This is important because we need to make sure that we can remove old
ACK ranges from the received packet history. The logic we implemented
before was not correct, since we only made sure that every 20th packet
would be retransmittable, but we didn't have any guarantee that this
packet would also contain an ACK frame.
