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

and fix the bug on the ```FutureRequestExecutionService``` (unexpected closing)
Just to be sure: you want to keep the builder and add a factory, to hide the build of the ```SimpleHttpClient``` object, right?

I'm hesitating to change my code again: your proposal is interesting, but I'm not sure it makes things really better or easier to understand.