One InitApp/App/AppendAppInit scheme to rule them all

[dblohm7]

Per this discussion, we still have room for improvement concerning app init. Ideally we'd have walk.App() callable from within appInits, and still have some sort of mutual exclusion around InitApp and App calls, and have App be consistently safe to call from any goroutine.

[dblohm7]

Also App() is currently returning non-nil even if InitApp failed.