rewrite ws client to expose a callback instead of a channel

callback gives more power to the publisher. plus it is optional
comparing to a channel, which will block the whole client if you won't
read from it.

rpc/lib/client/ws_client_test.go

@@ -186,8 +186,6 @@ func callWgDoneOnResult(t *testing.T, c *WSClient, wg *sync.WaitGroup) {
 			if err != nil {
 				t.Fatalf("unexpected error: %v", err)
 			}
-		case <-c.ReconnectCh:
-			t.Log("Reconnected")
 		case <-c.Quit:
 			return
 		}