Spec: acknowledge format

[berkus]

Need to specify the format of ACK frame.

• we pick the packet-based acks
• verify it is sound
• make graphical explanations for edge cases
• write tests based on logical assertions of the invariants
• build and run ACK-only communications test showing stream progress as function of packet loss rate
  • single stream
  • multiple streams

[berkus]

Tests tracked in metta-systems/libsss#1