Create framing code

[berkus]

Packet assembly from all streams that require it

• Streams waiting to send data
• Ack and priority change events
• Decongestion information frames sent periodically (depending on channel's decongestion strategy)

Some of this state is coming from streams (stream data and priority change), some from channel (acks) some from channel decongestion strategy that is variable per-channel (decongestion frame).

Spec 4.3