High level API for Web Transport

[huitema]

The current model requires applications to do a lot of plumbing. Ideally, we should have a simple way to run the loop in a background thread, while providing a simple API to use the most important features from other threads in the application:

• open a connection, and set parameters
• open streams
• queue data for sending in a stream, or on datagrams,
• receive queued data on stream or datagrams,
• close streams
• reset streams
• ask to stop sending

Adopting a simple queuing model would isolate implementations from the complexity of managing a packet loop.