Autoscaler concurrency limiter to avoid overloading

[jrasell]

Is your feature request related to a problem? Please describe.
Currently the internal autoscaler loops over the stored policies and will trigger and autoscaling run without any limit on the number of concurrent processes. In environments with a decent number of running and scalable jobs, this can cause pressure on the Nomad servers due to the number of API calls required to make.

Describe the solution you'd like.
The autoscaler should have some method to limit the number of concurrent autoscaling activities that can take place. This ideally would be something that has a sensible default, but that can be overridden by the operator. The Nomad num_schedulers provides an ideal reference.