Issue-740: Limit the queue size in the default ThreadPool in the SimpleHttpClient

and fix the bug on the ```FutureRequestExecutionService``` (unexpected closing)
It's possible that these values may need environment-specific tuning. I'd suggest we make that happen. Most adopters would rather change a few property values, rather than create new executors in the xml config. We just then ship with sensible defaults and let folks decide what the right value should be.