Improve [email protected] API

[briansmith]

RE: https://terrapin-attack.com/.

• Document the hazards in using the old API.
• Ensure that we enforce (and test the enforcement of) the requirement "Sequence numbers are never reset for a connection but roll over to 0 after 232 −1. To avoid replay attacks, rekeying must occur at least once every 232 packets".
• Try to redesign the API, perhaps based on NonceSequence, to avoid the hazards.
• Rename the old API so the hazards cannot be ignored.