Draft: Update Prop#324 for Flow Control improvements

• Specify rate advertisement of edge drain rate in XON, to minimize chatter
• Limit the frequency of XON/XOFF with consensus parsams
• Describe dropmark defenses using XON/XOFF limits
• Describe how half-closed edge connections are handled with flow control
• Describe flow control consensus parameters
• Describe flow control shadow experiments and live comparison
• Create and describe additional consensus parameters that will influence congestion control performance and memory usage
• Clarify performance metrics involved in experiments
• Remove some stale XXXs and TODOs