client: reset backoff to 0 after a connection is established (#2669)

#2663
#2636
diff --git a/clientconn.go b/clientconn.go
index edbed87..df1bb94 100644
--- a/clientconn.go
+++ b/clientconn.go
@@ -1059,6 +1059,7 @@
 				continue
 			}
 
+			backoffFor = 0
 			ac.mu.Lock()
 			reqHandshake := ac.dopts.reqHandshake
 			ac.mu.Unlock()