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

and fix the bug on the ```FutureRequestExecutionService``` (unexpected closing)
Similar to my comment above, rather than calling `init(null)`, you'd just call init with the appropriate parameters received from the ctor. So you'd be overloading the init method as well. 

Another alternative would be to come up with `SimpleHttpClientBuilder` class that is passed, and that would contain everything `SimpleHttpClient` needs. All settings would be injected into the builder instead which would possibly allow for less overloaded methods.