Add ability to tune the max number of cores to use when Avian is clustering.

[ispyhumanfly]

All of the popular Process Managers offer the clustering capability. PM2, StrongLoop, Forever, support applications not designed for clustering, and will expand across max CPUs to provide load balancing and failover across cores. Avian should make --cluster a first class option. Where we allow someone to NOT have to use clustering. Or, if using clustering, you can set the max number of workers.

[ispyhumanfly]

@Logikgate has proposed some ideas like, clustering is on by default, and, assumes generating a number of workers based on the total number of available cores. This is what we do now. What he's suggesting is that perhaps something like --cluster-max, or --cluster-max-workers, or --cluster-workers, or --cluster-cores is how one could tune Avian down to a set number of cores.