Represents a limit on a number of running requests.
This is basically a counter with a configured set of limits, that is used by {@link RequestLimitingHandler}.
When the number of active requests goes over the configured max requests then requests will be suspended and queued.
If the queue is full requests will be rejected with a 513.
The reason why this is abstracted out into a separate class is so that multiple handlers can share the same state. This allows for fine grained control of resources.
@author Stuart Douglas
@see RequestLimitingHandler