feat(transport): Change channel connect to be async (#107)

This makes it so you can check if the initial connection
is established. Before this we used reconnect which would
lazily attempt to connect. So if you were trying to connect
to a non existant Server you wouldn't find out until after
you attempted your first RPC. This simplifies everything
by allowing you connect before creating the RPC client.

BREAKING CHANGE: `Endpoint::channel` was removed in favor of
an async `Endpoint::connect`.

tonic-interop/src/bin/client.rs

@@ -41,7 +41,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         );
     }
 
-    let channel = endpoint.channel();
+    let channel = endpoint.connect().await?;
 
     let mut client = client::TestClient::new(channel.clone());
     let mut unimplemented_client = client::UnimplementedClient::new(channel);