Update logging to use core/async.111.28.00 logging API

As of core/async.111.25.00, `Log.of_lazy` is no longer part of package's public API. In 111.28.00, the `Log.level` call was allowing users of the package to implement `of_lazy` without to manage the log level manually. In addition, new optional arguments were introduced along with a `Log.string` call. This is all now part of the public API of the `Async_OpenFlow` module.

Closes #148.