Occasional shutdown noise from KafkaTasksExecutionTriggerer

Calling `executorService.submit()` [here](https://github.com/transferwise/tw-tasks-executor/blob/5b4777dde2ea089caa2169b868932fcd07feb371/tw-tasks-core/src/main/java/com/transferwise/tasks/triggering/KafkaTasksExecutionTriggerer.java#L624) after it has been asked to shut down leads to a `RejectedExecutionException`, which ends up logged as an error.

See https://rollbar.com/Wise/fin/items/19379/