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:
- [x] test (does it really work)?
- [x] PHP Doc

How to test:
1. Open the search dialog and enter 2 characters
2. Note the ajax response time and compare it to a run without this PR. Make sure the cache is emptied inbetween the runs (e.g. set cache TTL to a low value in LDAP settings, e.g. 10 seconds. Don't set it to 0, which disables the cache).