gplazma-ldap: support uniqueMember based group membership query

support group membership for LDAP schema defined by RFC2307 and RFC2307bis

Acked-by: karsten Schwank
Target: master, 2.11, 2.10
Require-book: no
Require-notes: yes
(cherry picked from commit b53c69bbd54a73060b45bc8d86ee14c764fba865)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>