airframe-http: Deprecate HttpFilter[Req, Resp, F[_]]

[xerial]

The current request router depends on the legacy interface HttpFitler and HttpBackend, which accept arbitrary Req, Resp, F (Future) types.

Now that we have the standard Request/Response objects and Rx for async interface, no longer need to use these type abstractions and adaptors. The implementation of airframe-http will be much simpler.

[xerial]

This dependency is only airframe-http-finagle, so moving this implementation to airframe-http-finagle can be a safer option without introducing too much breaking changes.