Adjust meek upstream flow optimization

- Use as default the value that performed best in testing

psiphon/server/meek.go

@@ -82,7 +82,7 @@ const (
 	MEEK_MAX_SESSION_ID_LENGTH                       = 20
 	MEEK_DEFAULT_TURN_AROUND_TIMEOUT                 = 10 * time.Millisecond
 	MEEK_DEFAULT_EXTENDED_TURN_AROUND_TIMEOUT        = 100 * time.Millisecond
-	MEEK_DEFAULT_SKIP_EXTENDED_TURN_AROUND_THRESHOLD = 16384
+	MEEK_DEFAULT_SKIP_EXTENDED_TURN_AROUND_THRESHOLD = 8192
 	MEEK_DEFAULT_MAX_SESSION_STALENESS               = 45 * time.Second
 	MEEK_DEFAULT_HTTP_CLIENT_IO_TIMEOUT              = 45 * time.Second
 	MEEK_DEFAULT_RESPONSE_BUFFER_LENGTH              = 65536