feat(server): introduce Accept trait

The `Accept` trait is used by the server types to asynchronously accept
incoming connections. This replaces the previous usage of `Stream`.

BREAKING CHANGE: Passing a `Stream` to `Server::builder` or
  `Http::serve_incoming` must be changed to pass an `Accept` instead. The
  `stream` optional feature can be enabled, and the a stream can be
  converted using `hyper::server:🉑:from_stream`.

Cargo.toml

@@ -69,6 +69,11 @@ runtime = [
     "tokio-net",
     "tokio-timer",
 ]
+
+# unstable features
+stream = []
+
+# internal features used in CI
 nightly = []
 __internal_flaky_tests = []
 __internal_happy_eyeballs_tests = []