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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taskExecutor.execute is still used in the processNioSelections method of AbstractConnectionFactory. It creates IO threads used for reading from sockets. CompositeExecutor is still important because it divides IO and assembler threads, which prevents a deadlock.