LDAP: put out fetching of user meta data into a fully tested class of it...

...s own and update them (mail, quota, etc.) directly after mapping. Fixes #7785 properly on master.

This includes a seperation of methods to retrieve information for a specific user from the general unspecific user backend class and does not "pollute" the access class as in the small fix for stable6. Ships with unit test.

Please review (and if possible test :D) @uka-support @leo-b @bantu or others, please.