The scheduling policy, either cluster.SCHED_RR for round-robin or
cluster.SCHED_NONE to leave it to the operating system. This is a
global setting and effectively frozen once either the first worker is spawned,
or .setupPrimary() is called, whichever comes first.
SCHED_RR is the default on all operating systems except Windows.
Windows will change to SCHED_RR once libuv is able to effectively
distribute IOCP handles without incurring a large performance hit.
cluster.schedulingPolicy can also be set through the
NODE_CLUSTER_SCHED_POLICY environment variable. Valid
values are 'rr' and 'none'.