INT-3433 commit

INT-3433: Fixed NIO deadlock (or timout if the CallerBlocksPolicy is used) with Bounded ThreadPoolTaskExecutor.

JIRA: https://jira.spring.io/browse/INT-3433

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I haven't looked at the commit yet, but we can't do `Executors.newCachedThreadPool()` - all task executors have to be configurable (e.g. on JEE containers you have to use the container's threads). This is why I created the `CompositeTE` to avoid having to change the schema to allow the injection of two executors. I will try to get to reviewing the commit today.