[quicwg/base-drafts] Packet number transform should be negotiated (#1296)

[Praveen Balasubramanian <[e-mail]>]

Packet number encryption has been proposed as a method to prevent ossification as well as improve privacy of connection migration. However there is a performance cost of doing the transform. There are deployments where ossification is not a concern (intra-DC, private network) and there are connections which are single path where privacy is not a concern, only ossification is. 

Hence packet number transform needs to be a negotiation with the caveat that the negotiated transform must meet the requirements for the deployment scenario otherwise the connection should be failed.
 
Packet number transform can be negotiated using transport parameters. Following PN transforms should be supported:

1. Packet number encryption (PNE)
Connections expecting to use multiple paths due to gratuitous/voluntary migration MUST negotiate and use PNE transform. See issue 1271 where options are being discussed on how migration could be negotiated. If a client or server fails to negotiate PNE and negotiates connection migration, fail the connection.
2. Low CPU cost ossification prevention transform (like shuffle)
For connections on the Internet on single path require them to negotiate this transform. If client or server fails to negotiate this transform on the Internet, fail the connection. 
3. No-op (current draft) 
For intra-datacenter or single domain private network scenarios allow usage of the no-op transform. This is for constrained deployments so clients should not negotiate this transform by default.

Servers are free to negotiate PNE all the time. This allows server deployments which have no performance concerns to always use PNE. 



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