spanner: further reduce min retry backoff

This CL further reduces the min retry backoff from 100ms to 20ms, per
recommendation of spanner team.

Updates #1215.

Change-Id: I3311de21bdc100fb48d78e40caebc6edc7506e83
Reviewed-on: https://code-review.googlesource.com/c/38210
Reviewed-by: Chris Broadfoot <cbro@google.com>
diff --git a/spanner/internal/backoff/backoff.go b/spanner/internal/backoff/backoff.go
index 3ea2b38..773856f 100644
--- a/spanner/internal/backoff/backoff.go
+++ b/spanner/internal/backoff/backoff.go
@@ -23,7 +23,7 @@
 
 const (
 	// minBackoff is the minimum backoff used by default.
-	minBackoff = 100 * time.Millisecond
+	minBackoff = 20 * time.Millisecond
 	// maxBackoff is the maximum backoff used by default.
 	maxBackoff = 32 * time.Second
 	// jitter is the jitter factor.