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

and fix the bug on the ```FutureRequestExecutionService``` (unexpected closing)
Default sizing is hard. I know from usage patterns at our institution thousands of users log in at the start of the hour, which results in a flood of logout requests 10 hours later. Obviously this wouldn't suffice. I suppose that kind of pattern is less likely for the default sliding expiration TGT policy, but thought it was worth pointing out. In any case I would think a 5 or 10 deep queue provides a better balance.