JIRA: https://jira.spring.io/browse/INT-4379
- shutdown the container when the gateway is stopped
Also, improve test suite
- at the end of the tests, hundreds of threads are running, some caused by the above
but others because `TaskExecutor`s are not shut down
- reduce the number of iterations in the JMS pipeline tests to speed things up
- change more tests to extend `ActiveMQMultiContextTests`, to keep a single broker up
__cherry-pick to 4.3.x__ (perhaps just the gateway fix)