Refactor port_default_packet_handler to allow for AEAD validation before we have a channel

[nhorman]

Current, we do some amount of header parsing in port_default_packet_handler, for things like version negotiation and server address validation. But these checks occur before we have done AEAD validation on a packet, meaning we are somewhat susceptible to corruption of the header data, and may accept invalid values from a peer. Currently we are mitigating this through various checks and indpependent validations (i.e. checking the token-encoded dcid against the header dcid).

A better solution would be to refactor when we do AEAD validation to ensure that the packet has some level of integrity. This is a significant work effort however, and so should be undertaken post 3.5

I believe @Sashan has some ideas on how to accomplish this

[Sashan]

I think we need to go for it. We need to verify integrity of QUIC header before the packet is passed to channel. The integrity check must happen in port.