Fix setting inheritance

Looking for feedback on this, but a recent issue was brought up with "trust proxy" and got me reading through a bunch of old issues on here.

Personally I like having apps as isolated islands, but sometimes it's nice/makes sense to inherit some settings from the parent app when using sub apps. There was a suggestion back in https://github.com/strongloop/express/issues/905#issuecomment-4750824 to add "inherit ___" settings where you would pick and choose settings to inherit. This seems kludgy to me, but I guess works?

Really, right now the way mounted sub apps work with settings is half-assed :) They pretend like they will inherit settings, unless they are special magical default Express settings. You can't have it both ways, Express!

So, what should we do for 5.0:
1. Remove setting inheritance and make it opt-in only
a. per setting
b. any setting
2. Fix setting inheritance to differentiate "default settings" from Express itself vs user-set settings. When sub apps are mounted, all non-user-set settings will thus be inherited (which means default Express settings would now be inherited)