class LoggingEventLoop

`LoggingEventLoop` provides a subset of the `RealLoop` API, and adds logging.

Feel free to expose more of the `RealLoop` API as needed.

Note: the Google C++ style guide requires `public` inheritance. But

1. Public inheritance would allow callers to skip logging (by casting to the base

class), and/or if this class doesn't override all the overloads of the base

class (the overloads are complicated by `default` parameters), and

2. Composition won't work here, because the methods that `LoggingEventLoop`

needs to call are `protected` in the base class.

It _is_ possible to meet this needs while satisfying the style guide. A helper

class could inherit from `RealLoop`, and `LoggingEventLoop` could instantiate that

class and forward calls to it. But that adds more code without a meaningful benefit.