Make the fairness semantics of the ThrottleRequestFilter configurable

The default fairness setting for j.u.c.Semaphore is false for a good reason. Don't force the users of the ThrottleRequestFilter to use a fair j.u.c.Semaphore. 

[1] mentions a fairness penalty of nearly two orders of magnitudes.
So "Don't pay for fairness if you don't need it" [1]

[1] http://goo.gl/FUbfHt p.284