LDAP re-query DNS

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Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/174 and ZD issue https://gitlab.zendesk.com/agent/tickets/13591.

In order to improve the LDAP connection handling, it would be nice if we make sure that the DNS is re-queried. In many corporations, they advertise more than one IP on the LDAP name so if one has failed we should do another lookup and use another. This might happen transparently when we re-establish a connection but we should see for certain.