pacer: default the Google pacer to a burst of 100 to fix gcs pacing

Before this change the pacer defaulted to a burst of 1 which mean that
it kept being activated unecessarily.

This affected Google Cloud Storage and Google Photos.

See: https://forum.rclone.org/t/no-traverse-too-slow-with-lot-of-files/29886/12

lib/pacer/pacers.go

@@ -219,7 +219,7 @@ type GoogleDriveOption interface {
 func NewGoogleDrive(opts ...GoogleDriveOption) *GoogleDrive {
 	c := &GoogleDrive{
 		minSleep: 10 * time.Millisecond,
-		burst:    1,
+		burst:    100,
 	}
 	c.Update(opts...)
 	return c