Following the standard practices we've applied to Derecho, #includes for
headers within the Cascade library should use quotes rather than angle
brackets and appear in a separate group before all non-Cascade includes.
The Derecho includes should also be grouped before the standard library
includes, since Derecho is more like a related sub-component of Cascade
than a third-party library (they share a namespace).

This is a manual cherry-pick of commit
5758256 from the cascade_chain branch,
where I already made this change a while ago.