File issues to clarify STREAM behavior

[sappenin]

The following clarifications need to be made in the STREAM RFC and implementations:

Java Issues

• File issue to Ignore any new ConnectionAssetDetails frames after the first one (see Ignore ConnectionAssetDetails after the first one in a Connection. #326)

Rust Issues

• File issue to start sending ConnectionAssetDetails from the StatelessStreamReceiver (see STREAM Receiver: Start sending ConnectionAssetDetailsFrame on first connection interledger/interledger-rs#395).
• File issue to add ConnectionAssetDetails to the SPSP response (see Add AssetDetails data to SPSP Response interledger/interledger-rs#396)

RFC Issues

(Instigating Issue: interledger/rfcs#546)

• Update the RFC to indicate why CAD is optional (e.g., This is not needed for a Web monetization connection.)
• Update the RFC to indicate that only a single ConnectionAssetDetails should be accepted per connection, with the rest being ignored (Assume new ILP addreses don't change asset code/scale.)
• Update the RFC to clarify the definition of a Connection.

[sappenin]

This is complete per the links above.