INT-3410 TCP NIO Deadlock with Bound TE

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.