Eliminate race on ac.acbw (#1666)

Guard ac.acbw with mutex
diff --git a/balancer_conn_wrappers.go b/balancer_conn_wrappers.go
index c673b98..ebfee4a 100644
--- a/balancer_conn_wrappers.go
+++ b/balancer_conn_wrappers.go
@@ -246,7 +246,9 @@
 			return
 		}
 		acbw.ac = ac
+		ac.mu.Lock()
 		ac.acbw = acbw
+		ac.mu.Unlock()
 		if acState != connectivity.Idle {
 			ac.connect()
 		}