utACK (should solve #3182)

utACK.

@fanquake I looked around a bit for APIs we should be using, rather than disabling the functionality completely. I assumed that there would be some calls we could make before/after initiating some network activity (not that it would be useful for us, probably, given the nature of p2p), but I was disappointed in the documentation.

Basically, I would like to know how a network-heavy application should work with AppNap without disabling it, if only for my own edification. Have you stumbled onto anything along those lines?

@theuni It looks the like we'd be using methods out of the NSProcessInfo Class.

beginActivityWithOptions:reason:
endActivity:
performActivityWithOptions:reason:usingBlock:

They allow you to temporarily suspend idle sleep, display sleep, sudden termination, and automatic termination for the duration of a particular operation as well as allow an app to temporarily increase the scheduler timer precision while it is performing latency-critical operations.

Then we'd pick the right option flags for what we'd like to do, something like

NSActivityIdleSystemSleepDisabled
NSActivitySuddenTerminationDisabled
NSActivityLatencyCritical

@theuni The problem is not network-heavyness but the CPU heaviness of validating blocks. People complain that app nap slows down the sync due to that. Maybe it could re-enable it after it deems itself up to date.