Change class mappings from class to fqn-String

[cerker]

Currently, the SessionManager holds a map with Class as key and BapiMapping as value. In applications where Bapi annotated classes reside in different modules which are loaded by different classloader this can lead to problems, e.g. multiple EAR deployments in an application server (like JBoss >= version 7 or Wildfly) with a central module doing the Hibersap configuration.

The map can be changed to have a String with the fully qualified class name. The disadvantage of this change is that we now rely on unique fully qualified class names for @bapi annotated classes. Having different classes with the same fqn should be considered a design flaw anyway, so this should not be a big problem.