The original `Connect` trait had some limitations:
- There was no way to provide more details to the connector about how to
connect, other than the `Uri`.
- There was no way for the connector to return any extra information
about the connected transport.
- The `Error` was forced to be an `std::io::Error`.
- The transport and future had `'static` requirements.
As hyper gains HTTP/2 support, some of these things needed to be
changed. We want to allow the user to configure whether they hope to
us ALPN to start an HTTP/2 connection, and the connector needs to be
able to return back to hyper if it did so.
The new `Connect` trait is meant to solve this.
- The `connect` method now receives a `Destination` type, instead of a
`Uri`. This allows us to include additional data about how to connect.
- The `Future` returned from `connect` now must be a tuple of the
transport, and a `Connected` metadata value. The `Connected` includes
possibly extra data about what happened when connecting.
BREAKING CHANGE: Custom connectors should now implement `Connect`
directly, instead of `Service`.
Calls to `connect` no longer take `Uri`s, but `Destination`. There
are `scheme`, `host`, and `port` methods to query relevant
information.
The returned future must be a tuple of the transport and `Connected`.
If no relevant extra information is needed, simply return
`Connected::new()`.
Closes#1428
8c52c2dfd3