Allow setting nice level and proc affinity w/psutil

@Yasumoto @rgs1 

Adding psutil as a requirement, and using it to set niceness level and CPU affinity. CPU affinity will allow us to pin the process to a specific core as scapy can be CPU intensive at high packets per second load. In production, this will let us sandbox the monitoring code away from the Zookeeper daemon itself.
Looks like we should call `cpu_affinity` with an argument: https://github.com/giampaolo/psutil/blob/86668f711584df7d6c54689883f15786a4c2d07f/psutil/__init__.py#L673