Modified ajaxManager to now dequeue waiting requests when an active request completes.

- Also added a maxConcurrentSends function to signalR.transports to allow configuration of the ajaxManager.

#2456
Modified tests who monkey patch ajax for detective purposes to correctly monkey patch it.

- This did not matter before because we never relied on the return value within the SignalR layer (now we do).
- Also found that joining too many groups resulted in us quickly going over our URL limit of the browser, therefore decreased some of the request thresholds of tests that stress how many ajax requests can be made.

#2456