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/tests.rs

@@ -207,4 +207,3 @@ fn checkout_win_allows_connect_future_to_be_pooled() {
     }
 }
 
-