[WFLY-3072] Add support for referral handling for LDAP within security realms.

This change was originally scheduled for WildFly 8.0.1 so has targeted the 2.1 schema, however the size of the changes has grown to cover various scenarios so if needed I could bump this to WildFly 9.

Would say however this change is to add something we never supported before and includes tests for the new scenarios - the existing test cases for LDAP support are functionally unmodified so existing behaviour has remained the same.