fix(everything) log.Debug -> log.Debugf

```
gofmt -r 'log.Debug -> log.Debugf' -w .
```

@whyrusleeping @jbenet 
Yeah. It's my understanding that Debugf("a string") is functionally equivalent to Debug("string") but not the other wat around. 

Ideally, Debug isn't even in the interface so we guard against programmer error.