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-nats.go

@@ -127,7 +127,7 @@ func closeNATS(conn natsIOConn) {
 }
 
 func newNATSNotify(accountID string) (*logrus.Logger, error) {
-	natsL := serverConfig.GetNATSNotifyByID(accountID)
+	natsL := serverConfig.Notify.GetNATSByID(accountID)
 
 	// Connect to nats server.
 	natsC, err := dialNATS(natsL, false)