Update LookupValue fqdn matchers when a host is renamed, fixes #6358

even though we used `Proc.new { |host| ` in much of the code, I'm pretty sure it can just be written as:
```ruby 
after_update :update_lookup_value_fqdn_matchers, :if => 'fqdn_changed?'
```
Also, I think `after_update` is the same as `after_save :on => :update`