* Clean up some of protocol abstractions
- Renamed PipeConnection to DuplexPipe
- Removed MemoryPool from ConnectionContext
- Work around value tuple issue on net471
- Most of the shared code is in the HttpProtocol class (former Frame)
- Virtual calls handle protocol-specific things
- Move the ProcessRequestsAsync loop to HttpProtocol
- Implement HTTP/1.x request processing in Http1Connection and HTTP/2
in Http2Stream, with Http1Connection<T> and Http2Stream<T> subclassing
those classes in order to handle the generic HttpContext parameter
- Split MessageBody into Http1MessageBody and Http2MessageBody,
with MessageBody containing shared member variables and methods
* Improve ConnectionLimitMiddleware and connection pipeline building
* Add IDecrementConcurrentConnectionCountFeature
* Flow connection features from connection middleware
- Use the pipe events and removed the Tasks from ConnectionContext
- Remove OnConnectionClosed from FrameConnection. Since the `FrameConnetion` is a single middleware, not the entire pipeline, we shouldn't need to wait on the connection close there.
- It seems like the callbacks are rooted on the pipe even after they fire. This needs to be investigated in pipelines.
* Added initial connection middleware pipeline
- Implemented IConnectionBuilder on ListenOptions. Kept IConnectionAdapter for now.
- Delay the configure callback for ListenOptions until the server has started.
- Added ConnectionLimitMiddleware and HttpConnectionMiddleware
- Expose ConnectionAborted and ConnectionClosed on ConnectionContext and
IConnectionTransportFeature
- Updated the tests
- Removed IConnectionApplicationFeature
- Moved Application to IConnectionTransportFeature