maximize report publishing timeout

If the app really does take a long time to process reports, it is
better not to time out and send it more reports. However, we do want
to send at least one report per app.window, otherwise the scope UI
will go blank.

Fixes #2464 (as much as is practically possible)

probe/appclient/app_client.go

@@ -19,7 +19,7 @@ import (
 )
 
 const (
-	httpClientTimeout = 4 * time.Second
+	httpClientTimeout = 12 * time.Second // a bit less than default app.window
 	initialBackoff    = 1 * time.Second
 	maxBackoff        = 60 * time.Second
 )