Makes server manager shift away from failed servers from Serf events.

Because this code was doing pointer equality checks, it would work for
the case of a failed attempted RPC because the objects are from the
manager itself:

https://github.com/hashicorp/consul/blob/v1.0.3/agent/consul/rpc.go#L283-L302

But the pointer check would always fail for events coming in from the
Serf path because the server object is newly-created:

https://github.com/hashicorp/consul/blob/v1.0.3/agent/router/serf_adapter.go#L14-L40

This means that we didn't proactively shift RPC traffic away from a
failed server, we'd have to wait for an RPC to fail, which exposes
the error to the calling client.

By switching over to a name check vs. a pointer check we get the correct
behavior. We added a DEBUG log as well to help observe this behavior during
integrated testing.

Related to #3863 since the fix here needed the same logic duplicated, owing
to the complicated atomic stuff.

/cc @dadgar for a heads up in case this also affects Nomad.

agent/router/manager.go

@@ -256,7 +256,7 @@ func (m *Manager) NotifyFailedServer(s *metadata.Server) {
 	// the server to the end of the list.
 
 	// Only rotate the server list when there is more than one server
-	if len(l.servers) > 1 && l.servers[0] == s &&
+	if len(l.servers) > 1 && l.servers[0].Name == s.Name &&
 		// Use atomic.CAS to emulate a TryLock().
 		atomic.CompareAndSwapInt32(&m.notifyFailedBarrier, 0, 1) {
 		defer atomic.StoreInt32(&m.notifyFailedBarrier, 0)
@@ -267,9 +267,10 @@ func (m *Manager) NotifyFailedServer(s *metadata.Server) {
 		defer m.listLock.Unlock()
 		l = m.getServerList()
 
-		if len(l.servers) > 1 && l.servers[0] == s {
+		if len(l.servers) > 1 && l.servers[0].Name == s.Name {
 			l.servers = l.cycleServer()
 			m.saveServerList(l)
+			m.logger.Printf(`[DEBUG] manager: cycled away from server "%s"`, s.Name)
 		}
 	}
 }