Duplicates

performs case-insensitive comparisons for DOIs, fixes some cases where ISBNs were not being compared correctly, and reduces number of DB queries to 2.

Currently, on my laptop, this does not really improve the performance that much, but I think if Zotero moves to async DB queries, this should simplify and speed things up quite a bit. So if the second commit is a bit premature, I'll throw it out.

BTW, is anyone working on adding async DB access methods to Zotero.DB?