Split out checker into pure api module and stand alone application

The repository checker module recently was integrated into the repository engine and repository test modules. One of the consequences of this change is that the repository engine now pulls in a different jackrabbit, jcr, and lucene version and even a mysql connector dependency. These dependencies are indeed needed by the checker as standalone application. Another problem is that the checker jar packages a log4j file.

We need to separate the checker module into a standalone application and an embeddable module.