LDAP users are deleted when server is offline

Summary

When an LDAP server is offline for an extended period (24 hours+), users from that server are deleted from GitLab.

Steps to reproduce

1. Enable LDAP sign in
2. Add several LDAP users
3. Take the LDAP server offline for more than a day
4. The synced LDAP users will be deleted from GitLab

Example Project

What is the current bug behavior?

LDAP users are deleted from GitLab when the LDAP server is offline for an extended period of time.

What is the expected correct behavior?

Users should not be removed from GitLab just because the LDAP server has gone offline.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes