MXS-1929: Prevent destruction of services with filters

If a service has filters, they must first be removed before the service
can be destroyed. This is not a functional requirement but it keeps the
behavior consistent so that the relationships of a service must be empty
before it is destroyed.

server/core/service.cc

@@ -889,6 +889,11 @@ bool service_can_be_destroyed(Service *service)
         }
     }
 
+    if (!service->get_filters().empty())
+    {
+        rval = false;
+    }
+
     return rval;
 }