clean up dial functions

* add a context to all Dial functions
* remove the explicit Dial{*}Context functions

integrationtests/self/early_data_test.go

@@ -50,6 +50,7 @@ var _ = Describe("early data", func() {
 		defer proxy.Close()
 
 		conn, err := quic.DialAddr(
+			context.Background(),
 			fmt.Sprintf("localhost:%d", proxy.LocalPort()),
 			getTLSClientConfig(),
 			getQuicConfig(nil),