allow to search for "auxiliary" objectClasses (i.e. those in ldap_entry_objclasses); allow to return all objectClasses including "auxiliary"; various cleanup

including:
- fixes according to new API changes
- closing db connection in connection_destroy callback, not unbind
- support of new schema code, samples changed accordingly
- support for multiple objectclasses (to distinguish from unique objectclass-to-tables mapping)
- auto 'ref' attribute support
- samples now include illustrations of using these 2 features to make named referrals as described in ldapext-namedref draft

more to come:
- documentation update
- different improvements to be more close to native directory (after beta?)