When QUIC abandoned the "stream 0" design, it introduced separate
packet number spaces for packets with different encryption levels.
Packet number compression now also works per packet number space.
The current code doesn't lead to any problems if the peer starts sending
with packet number 0, as we only exchange a few packets in the Initial
and the Handshake packet number space and there's nothing to compress.
It might lead to problems if the peer starts with a large packet number
in one space (which is allowed by the spec), and then starts with a
small packet number in another packet number space.
