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JIRA: https://jira.spring.io/browse/INT-3410 When running a fixed thread pool with a bound queue, and CALLER_RUNS execution rejection policy, it was possible to deadlock the IO selector thread. Add a `CompositeExecutor` to use different threads for IO to those used for message assembly. Add a `CallerBlocksPolicy` to block the invoking thread (for a specified time) if the pool is exhausted. Add documentation. Polishing