Currently LoggerContext uses LogbackMDCAdapter.

There doesn't seem to be much need for it to do that rather than just using the MDCAdapter interface.

Using LogbackMDCAdapter ties it to the implementation and makes it difficult to replace, whereas using the interface would make it easy to replace.

As a Vertx user the thread-local storage of MDCAdapter isn't a useful implementation for me, so I'd like to be able to replace it.