LDAP: central function for user and group list retrieval (will replace retrieveList from group part)

LDAP rewrite, use unique LDAP user and group identifiers on LDAP side as well as fancy (unqiue as far as users a know from LDAP) names on the ownCloud side. It's done via mapping of owncloud names and LDAP identifiers.

some performance enhancements: faster searching for users and groups in their specific subtrees. Reading instead of searching were possible.

thanks for the feedback of Kevin van Kuik

use uniquemember instead of deprecated memberuid to identify LDAP group members. Attention: not every method is adjusted yet, so it breaks some things.