Add UI for entering imageinfo_cache_url_key

Actually this seems to work differently than I would expect. I would think you set a shared key on the server and client, and perform an access check on that key on the async worker before proceeding. It doesn't seem like that is the case? Or is the current assumption that you are passing async between systems that have the same backend DB? I would want to spin up cache-only servers with their own DB to minimize the Drupal memory load on the worker machines.

UI was easy enough... dropped this in to imageinfo_cache.admin.inc. Now I can set keys to match on servers with different backends.

  $form['imageinfo_cache_url_key'] = array(
    '#type'           => 'textfield',
    '#title'          => t('Shared authentication key'),
    '#default_value'  => variable_get('imageinfo_cache_url_key', FALSE),
    '#description'    => t('Use at least a 9 digit key. This must match on all async servers.'),
  );

Hmm... one of the other assumptions seems to be that you have a shared underlying filesystem between the primary server and async workers. It runs imagecache_generate_image($preset, $file->filepath); I would have thought it would return that to the caller but it writes it out. So I guess I need to mount the files dir between the two hosts.