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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So, how about just to go with two configured `taskExecutor`s and get rid of `CompositeExecutor` complexity. Assume `CallerBlocksPolicy` isn't needed, too.
Sorry, it isn't in my hands, because so low level TCP issues is out of my knowledges and I really need a test to play with it.
Otherwise we need @garyrussell input.

Thank you for your attention to this issue anyway