add backoff debug statement

backoff.go

diff --git a/backoff.go b/backoff.go
index b2310c7..57e2176 100644
--- a/backoff.go
+++ b/backoff.go
@@ -70,6 +70,7 @@
 		return b.max
 	}
 
+	log.Debugf("Backing off for %d ms", int64(dur/time.Millisecond))
 	return dur
 }