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

and fix the bug on the ```FutureRequestExecutionService``` (unexpected closing)
I'm not sure to see the difference between a builder and a factory: I would say that both have the responsability of building the appropriate objects.
The built objects are stored in the builder and retrieved from it.

They could be retrieved and kept by this class just after the initialization. Agreed?