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

and fix the bug on the ```FutureRequestExecutionService``` (unexpected closing)
I would recommend the following two ctors:

* One accepts a full executor as a single parameter
* One that constructs a ThreadPoolExecutor with the following values: 200, 200, 0L, TimeUnit.MILLISECONDS

We might also want to think about simply changing the default to be constructed with a ThreadPoolExecutor and 200, 200, 0L, TimeUnit.MILLISECONDS?