wtclient: schedule ForceQuit first during Stop

Currently the ForceQuit call is scheduled after trying to stop the
backup queue. In certain cases, the call to stop the queue never
finishes, which means the force quit is never scheduled. We rememdy by
scheduling this call before any other operations to ensure we can always
exit ungracefully if necessary.

watchtower/wtclient/client.go

@@ -462,13 +462,7 @@ func (c *TowerClient) Stop() error {
 	c.stopped.Do(func() {
 		log.Debugf("Stopping watchtower client")
 
-		// 1. Shutdown the backup queue, which will prevent any further
-		// updates from being accepted. In practice, the links should be
-		// shutdown before the client has been stopped, so all updates
-		// would have been added prior.
-		c.pipeline.Stop()
-
-		// 2. To ensure we don't hang forever on shutdown due to
+		// 1. To ensure we don't hang forever on shutdown due to
 		// unintended failures, we'll delay a call to force quit the
 		// pipeline if a ForceQuitDelay is specified. This will have no
 		// effect if the pipeline shuts down cleanly before the delay
@@ -483,6 +477,12 @@ func (c *TowerClient) Stop() error {
 			time.AfterFunc(c.cfg.ForceQuitDelay, c.ForceQuit)
 		}
 
+		// 2. Shutdown the backup queue, which will prevent any further
+		// updates from being accepted. In practice, the links should be
+		// shutdown before the client has been stopped, so all updates
+		// would have been added prior.
+		c.pipeline.Stop()
+
 		// 3. Once the backup queue has shutdown, wait for the main
 		// dispatcher to exit. The backup queue will signal it's
 		// completion to the dispatcher, which releases the wait group