Publicly export wildfly-clustering-api

The goal is to create a single publicly exported clustering module, with a refined, stable public API.
The current wildfly-clustering-api module is too cluttered.
Once the api module is cleaned up, we can merge it with the current wildfly-clustering-annotations module - which does not need to be separate.

As part of this, we will also clean up the wildfly-clustering-impl module. CoreGroupCommunicationService needs to be overhauled, and split into smaller, light-weight components. The distributed lock manager components are obsolete.