WIP: LDAP: optimize retrieval of displaynames, fixes #8135

Problem: determining the display name per users takes time if it is not cached, because it requires a query per user.

Solution: get the display name immediately when searching for the users and cache it. 

Funfact: half of the solution was already there, only the caching part was not.

TODOs left:
- [ ] test (does it really work)?