correct LDAP user count on setup with many users (occ user:report)

- Currently, the user count cannot be higher than the size limit on the LDAP server
- Paged Search will be done in future. 
 - OpenLDAP still follows the configured sizeLimit. Nothing we can do
 - AD counts all users now
 - other servers not tested

* [x] replace hard coded chunk size with ldapPagingSize from PR #6221, once it is merged
* [x] evaluate unit test on Access:count. Real conditions cannot be reached without test LDAP server though. Loop and addition can be ensured however…

Open for comments @leo-b @PVince81  @DeepDiver1975 or others

How to test?

1. configure access to an AD with more than 1k users that are accessible for ownCloud. Do not have other LDAP servers active (well, it works with others but then you need to apply maths ;) ).
2. run ```occ user:report``` in master. Total LDAP users will be 1000k (with default AD sizeLimit)
3. run ```occ user:report``` in this branch. Total LDAP users will be correct now.

Test will not work with OpenLDAP (see above)