log: Stop using `logrus.New()`

The ultimate end goal for our logging infrastructure refactorings is to configure a logger once, and then inject it into all places that need it. Functions like log.Default() are thus bound to eventually go away in favor of using either explicitly passed-down loggers or loggers which have been injected into the context.

This MR thus refactors code to stop using logrus.New() in favor of log.Default(), ctxlogrus.Extract() or testhelper.NewDiscardingLogEntry().