Add Pipeline conversions and rewrite internals with channels

[mschuwalow]

Reason to go this way is that for pipeline to work correctly, we need to ensure that the Publisher and the Subscriber run concurrently on different fibers. Imo this is easiest / cleanest by relying on the AsynInput abstractions baked into the channelexecutor.

resolves #364

[kciesielski]

Bumping this, as it would really help to implement WebSocket support for tapir-netty-zio, where we use netty-reactive-streams underneath and need a Processor[Req, Resp] created out of a ZPipeline. Any chance to push this a little bit forward, @runtologist?

[dispalt]

This PR also makes cancellations of streams work better, previously cancellations only seemed to complete the stream if read after closing the stream.