See if we can avoid spurious `ldap_blocked` issues

Zendesk: https://gitlab.zendesk.com/agent/tickets/19504

It seems that in some cases (not all that rare) communication between GitLab and LDAP is not possible for a short time. In this case, users can become temporarily ldap_blocked. @brodock fixed it so users will subsequently be unlocked on the next login. Now, can we see if we can avoid this situation altogether?

This is not urgent. It's a nice to have.