config: setter/getter for Notifier and Logger into its own struct. (#3721)

This is an attempt cleanup code and keep the top level config
functions simpler and easy to understand where as move the
notifier related code and logger setter/getter methods as part
of their own struct.

Locks are now held properly not globally by configMutex, but
instead as private variables.

Final fix for #3700

cmd/notify-elasticsearch.go

@@ -55,7 +55,7 @@ func dialElastic(esNotify elasticSearchNotify) (*elastic.Client, error) {
 }
 
 func newElasticNotify(accountID string) (*logrus.Logger, error) {
-	esNotify := serverConfig.GetElasticSearchNotifyByID(accountID)
+	esNotify := serverConfig.Notify.GetElasticSearchByID(accountID)
 
 	// Dial to elastic search.
 	client, err := dialElastic(esNotify)