Marked a function as obsolete and removed an obsolete test.
Added lending_identifier_s to solr and using it to find the checkout status in search results.

It is easier now to find a book is checkedout or not from ia identifier insted
of book key.
Switched everywhere to using ebooks/$identifier instead of ebooks/books/OLxxxM

That handles the issue of multiple editons with same ocaid and also plays
nicely with archive.org.