Search indexing using notifications

[dread]

Currently search indexing is triggered directly using a Postgresql db callback. Now take advantage of the Notification system to register interest in all package changes and db changes to trigger this instead.

The indexing shall run in a separate shell/process, managed by supervisord.

[pudo]

I am currently writing a Solr subclass for the search index (#317) and would propose adding standard methods to the ckan.libn.search.Search class: index_package(), index_tag(), index_group(). Those could then be called by a generic queue consumer, irrespective of the used search back-end. I will prototype such a consumer soon, so we should talk to avoid doing some duplicate work here.