- `ConsumerControlState` was replaced with `ResponseStreamState` in `HTTPRequestStateMachine`
- `channelRead`s are buffered up to a `channelReadComplete`.
- on `channelReadComplete` all buffered body parts are forwarded to the consumer
This commit adds an explicit `HTTPRequestStateMachine`. It is intended to be used in two instances in the future:
1. As a substate machine for a `HTTP1ConnectionStateMachine`, when the connection is in a request.
2. As a state machine for a `HTTP2RequestHandler` that operates on a single http2 stream created by the multiplexer.
The new code paths are not triggered today, but will be enabled in the future.