MM-12974: guard app plugins with mutex (#9818)

* guard app plugins with mutex

Shutting down the app could race with a goroutine that uses the plugins environment, since we shut down the plugins first before cleaning up goroutines.

* fix go vet issues

app/server.go

@@ -54,8 +54,9 @@ type Server struct {
 	goroutineCount      int32
 	goroutineExitSignal chan struct{}
 
-	Plugins                *plugin.Environment
+	PluginsEnvironment     *plugin.Environment
 	PluginConfigListenerId string
+	PluginsLock            sync.RWMutex
 
 	EmailBatching    *EmailBatchingJob
 	EmailRateLimiter *throttled.GCRARateLimiter