MM-34419: Fix plugin enable race (#17511)

A race happens when we try to enable plugins
in an HA environment. There is no loss in functionality
here, but it's a timing bug.

There are two bugs here.

The first one is a case of nested config listeners. a.InitPlugins
is called in InitServer, and as well as a config listener. And that
calls a.SyncPluginsActiveState(). But inside a.InitPlugins,
there is yet another config listener which again calls
a.SyncPluginsActiveState(). The first fix is to simply not call
the method again from the nested listener.

The second bug happens because the config changed message is sent
across the cluster only after saving the config locally
in the store. To fix this, we simply change GetPluginStatus
to not fail and return empty status when other nodes don't have plugins
enabled.

https://mattermost.atlassian.net/browse/MM-34419

```release-note
Fix a race condition where enabling plugins would result
in spurious errors in the logs.
```

app/plugin.go

@@ -210,9 +210,13 @@ func (a *App) InitPlugins(pluginDir, webappPluginDir string) {
 	// Sync plugin active state when config changes. Also notify plugins.
 	a.Srv().PluginsLock.Lock()
 	a.RemoveConfigListener(a.Srv().PluginConfigListenerId)
-	a.Srv().PluginConfigListenerId = a.AddConfigListener(func(*model.Config, *model.Config) {
-		a.installFeatureFlagPlugins()
-		a.SyncPluginsActiveState()
+	a.Srv().PluginConfigListenerId = a.AddConfigListener(func(old, new *model.Config) {
+		// If plugin status remains unchanged, only then run this.
+		// Because (*App).InitPlugins is already run as a config change hook.
+		if *old.PluginSettings.Enable == *new.PluginSettings.Enable {
+			a.installFeatureFlagPlugins()
+			a.SyncPluginsActiveState()
+		}
 		if pluginsEnvironment := a.GetPluginsEnvironment(); pluginsEnvironment != nil {
 			pluginsEnvironment.RunMultiPluginHook(func(hooks plugin.Hooks) bool {
 				if err := hooks.OnConfigurationChange(); err != nil {