vfs: change default --vfs-read-wait to 20ms

In my testing with local and remote storage this is a good compromise
between delaying the seeks and failing to wait for in sequence reads.

See: https://forum.rclone.org/t/constantly-high-iowait-add-log/14156/40

vfs/vfscommon/options.go

@@ -53,5 +53,5 @@ var DefaultOpt = Options{
 	CacheMaxSize:      -1,
 	CaseInsensitive:   runtime.GOOS == "windows" || runtime.GOOS == "darwin", // default to true on Windows and Mac, false otherwise
 	WriteWait:         1000 * time.Millisecond,
-	ReadWait:          5 * time.Millisecond,
+	ReadWait:          20 * time.Millisecond,
 }