feat(client): add ALPN h2 support for client connectors

- Adds `Connected::negotiated_h2()` method to signal the connection must
  use HTTP2. `Connect` implementations should set this if using ALPN.

If a connection to a host is detected to have been upgraded via ALPN,
any other oustanding connect futures will be canceled, and the waiting
requests will make use of the single HTTP2 connection.

The `http2_only` builder configuration still works the same, not
requiring ALPN at all, and always using only a single connection.

src/client/conn.rs

@@ -466,6 +466,7 @@ impl Builder {
         T: AsyncRead + AsyncWrite + Send + 'static,
         B: Payload + 'static,
     {
+        trace!("client handshake HTTP/{}", if self.http2 { 2 } else { 1 });
         Handshake {
             builder: self.clone(),
             io: Some(io),