Service trait evolution

[ximon18]

With the original Service trait single responses and streams were supported by a sync impl returning a custom Transaction type, with middleware implemented via MiddlewareProcessor, MiddlewareBuilder and MiddlewareChain type.s

PR #307 updates the Service trait to be async and return a Future<Output = Stream> with middleware implemented as a layered service approach ala Hyper.

But we know this is not where we want to go with this. We want to simplify the Service trait as much as we can. This issue captures the current state and plans going forwards.

Additional notes:

• In a comment on PR Service layering #307 partim described possible next steps.
• PR RFC 1035 zone maintenance support. #335 makes additional changes to support communication of additional custom state between service layers.
• @Philip-NLnetLabs has proposed splitting out streaming response support to simplify the Service trait for the main use case, and to handle streaming/XFR separately. An initial version of this can be seen in his PR Service layering #307.