ICS4: v2 handling of packet flow

[AdityaSripal]

In v2, the only functions that are necessary for 04-channel are the packet flow handlers. Should we rename this spec so it is no longer called channel?

• Retain only the packet flow handlers and ensure all similar checks are still made
• Specify and explain new packet structure (including multidata)
• Specify and explain new acknowledgment structure (including metadata)
• Specify and explain how the packet flow handler will route verification to the underlying client and route application data to all relevant applications

[sangier]

Closed by https://github.com/cosmos/ibc/pull/1148