Crash when qlogging a Version Negotiation packet

[marten-seemann]

See ipfs/kubo#7512.

It looks like qlog.Tracer.ReceivedVersionNegotiationPacket() is called after qlog.Tracer.Export() has been called.
This could happen when the session runs into a handshake timeout, and then receives the Version Negotiation packet at just the right moment.

We should handle Version Negotiation packets in the session, so we don't have a race condition between Export() and other logging calls.