Index ia: pages in solr when they are visited  for the first time.

When the page is visited for the first time, an entry is added to store
and solr-updates notices that and adds that to solr.
Added user-agent as openlibrary for all solr-update operations.

Useful to the ops team to identify who is making the requests.
Fix crash on /books/ia:xx pages when archive.org metatdata contains
empty tags.

Empty tags are represneted as {} in metadata API. Ignoring them to avoid
errors in OL.
When the metadata has multiple values, join them by new lines.
In search results, display cover from archive.org if a work has no cover and has iaid.
validate archive.org identifier before passing it to solr.
reduce the number of fetches from 100 to 10 in solr-updater.
Ignore jstor articles in solr-updater.
Exclude jstor- items from /books/ia:xxx pages