Support unknown record types (RFC 3597).
Fallback to generic LDAP attribute "UnknownRecord;TYPE256" if attempt to
add specific attribute like "URIRecord" failed with LDAP_OBJECT_CLASS_VIOLATION
and always delete both attributes like "URIRecord" and "UnknownRecord;TYPE256".
https://fedorahosted.org/bind-dyndb-ldap/ticket/157