Run at most one LDAP check at a time for a user

https://gitlab.zendesk.com/agent/tickets/16397 cc @dblessing @DouweM

The intended behavior of GitLab's LDAP checks is that once per hour (or some other configurable interval) GitLab queries the LDAP server to make sure its local user object is in sync with the LDAP server. This check is initiated by the GitLab user making a request.

We are getting reports (see ZD ticket above) that in some situations a user can trigger up to 10 simultaneous LDAP checks on the GitLab server. Because LDAP checks are relatively expensive I wonder if we should use a locking mechanism to make sure that only one LDAP check is running at a time for any given user. It would still be OK to do LDAP checks for 10 different users at a time.