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

and fix the bug on the ```FutureRequestExecutionService``` (unexpected closing)
You raise a good point. At first, I had the idea of allowing 20% of the total of the threads in the queue to be able to handle bursts. But I think it really depends on the way the CAS server is used, so I kept just one element.

It can be changed.