Geo: Secondary with LDAP configured can't update last_credential_check_at due to read-only DB

Noticed on a customer with a Geo secondary instance with LDAP configured:

{
  "method": "GET",
  "path": "/<REDACTED>/<REDACTED>.git/info/refs",
  "format": "html",
  "controller": "Projects::GitHttpController",
  "action": "info_refs",
  "status": 500,
  "error": "ActiveRecord::StatementInvalid: PG::ReadOnlySqlTransaction: ERROR: cannot execute UPDATE in a read-only transaction\n: UPDATE \"users\" SET \"last_credential_check_at\" = '2018-08-21 06:07:22.825034', \"updated_at\" = '2018-08-21 06:07:22.831368' WHERE \"users\".\"id\" = 7830",
  "duration": 487.65,
  "view": 0.0,
  "db": 12.63,
  "time": "2018-08-21T06:07:22.347Z",
  "params": [
    {
      "key": "service",
      "value": "git-upload-pack"
    }, 
    {
      "key": "namespace_id",
      "value": "<REDACTED>"
    }
  ]
}

/cc @akaemmerle @stanhu @abuerer