superfeedr has done a nice job with this backbone -> indexedDB adapter,
but their query interface is somewhat limited. This commit adds an
alternate interface that lets us specify the index and cursor bounds we
want. This interface requires deeper knowledge of indexedDB indices, but
is more powerful overall.
Getting up and running with IndexedDB was pretty easy, thanks to
backbone. The tricky part was making reads and writes asynchronous.
In that process I did some refactoring on Whisper.Threads, which
has been renamed Conversations for consistency with the view names.
This change also adds the unlimitedStorage permission.
