Use the DN to fetch group memberships from LDAP

fixes #9901

library/Icinga/Authentication/User/LdapUserBackend.php

@@ -364,7 +364,12 @@ class LdapUserBackend extends LdapRepository implements UserBackendInterface, In
                 return false;
             }
 
-            return $this->ds->testCredentials($userDn, $password);
+            $testCredentialsResult = $this->ds->testCredentials($userDn, $password);
+            if ($testCredentialsResult) {
+                $user->setAdditional('ldap_dn', $userDn);
+            }
+
+            return $testCredentialsResult;
         } catch (LdapException $e) {
             throw new AuthenticationException(
                 'Failed to authenticate user "%s" against backend "%s". An exception was thrown:',

library/Icinga/Authentication/UserGroup/LdapUserGroupBackend.php

@@ -532,6 +532,7 @@ class LdapUserGroupBackend /*extends LdapRepository*/ implements UserGroupBacken
      */
     public function getMemberships(User $user)
     {
+        if (($userDn = $user->getAdditional('ldap_dn')) === null) {
             $userQuery = $this->ds
                 ->select()
                 ->from($this->userClass)
@@ -545,6 +546,7 @@ class LdapUserGroupBackend /*extends LdapRepository*/ implements UserGroupBacken
             if (($userDn = $userQuery->fetchDn()) === null) {
                 return array();
             }
+        }
 
         $groupQuery = $this->ds
             ->select()